I had far more difficulty getting the generation of gif animations working correctly using ImageIO and Core-Graphics aka Quartz on MacOS/iOS than should have reasonably been expected.

main.m

//  main.m
//  creategif
//
//  Created by Kevin Meaney on 21/10/2013.

// I've briefly blogged about it here:
// http://blog.yvs.eu.com/2013/10/creating-gif-animations-using-coreimagequartz
// The following code is all that is all the code needed for a creating a command line tool to generate
// a gif animation from image files. 
// The main limitation with this code is that apart from the frame delay time of 0.1 second, every
// other frame delay time has to be a multiple of 0.5 seconds. There is no checking that the frame size
// of each image file is the same or resizing of frames to make them conform.
// I provide no warranty as to the operation of this code.

#import <Foundation/Foundation.h>
@import ImageIO;

void PrintUsage()
{
	printf("creategif - usage:\n");
	printf("	./creategif [-parameter <value> ...], filename1, filename2, ...\n");
	printf("	 Parameters are preceded by a -<parameterName>.  The order of the parameters is unimportant.\n");
	printf("	 All the input image filenames follow the parameters and their values.\n");
	printf("	 You probably need to escape or quote paths especially if the paths includes spaces etc..\n");
        printf("	 If the output file already exists then it will be overwritten if possible.\n");
	printf("	 Available parameters are:\n");
	printf("	 	-outputfile <output file path which should finish with a gif extension>\n");
	printf("	 	-delaytime <should be a floating point number above 0.1>\n");
//	printf("	 	-unclampeddelaytime <should be a floating point number above 0.1>\n");
	printf("	Sample export lines:\n");
	printf("	./creategif -outputfile blahblah.gif -delaytime 0.4 -unclampeddelaytime 0.4 image1.png image2.png image3.png image4.png\n");
	printf("	./creategif -outputfile \"Users/ktam/gifs/blahblah.gif\" -delaytime 1.0 image1.png image2.png image3.png image4.png\n");
}

BOOL GetFloatFromString(NSString *string, CGFloat *value)
{
    NSScanner *scanner = [[NSScanner alloc] initWithString:string];
    CGFloat floatVal;
    BOOL gotValue = [scanner scanDouble:&floatVal];
    if (gotValue)
    {
        *value = floatVal;
    }
    return gotValue;
}

NSMutableArray *inputImageList;
NSString *outFilePath;
CGFloat delayTime;
CGFloat unclampedDelayTime;
BOOL gotDelayTime = NO;
BOOL gotUnclampedDelayTime = NO;

bool GetArgs(int argc, const char **argv) // returns true if successful.
{
    bool success = true;
    
    argv++;
    argc--;
    const char *args;
    // Get the parameters
    while (argc > 0 && **argv == '-')
    {
		args = *argv;
        argv++;
        argc--;

        if ( ! strcmp ( args, "-outputfile" ) )
		{
            outFilePath = @(*argv++);
			argc--;
		}
        else if ( ! strcmp( args, "-delaytime" ))
        {
            gotDelayTime = GetFloatFromString(@(*argv++), &delayTime);
            argc--;
        }
/*        else if ( ! strcmp( args, "-unclampeddelaytime"))
        {
            gotUnclampedDelayTime = GetFloatFromString(@(*argv++), &unclampedDelayTime);
            argc--;
        }
*/
    }
    
    if (!(gotDelayTime || gotUnclampedDelayTime))
        success = false;

    if (!outFilePath)
        success = false;

    // If we've got to the end of arguments and there are no input files then bail.
    if (!success)
        return success;

    inputImageList = [[NSMutableArray alloc] initWithCapacity:0];
//    [inputImageList addObject:@(args)];
    // Finished getting parameters. Now get list of source files. 1 image per file.
    while (argc > 0)
    {
        args = *argv;
        argv++;
        argc--;
        [inputImageList addObject:@(args)];
    }
    if (!inputImageList || [inputImageList count] == 0)
        success = false;

    return success;
}

int main(int argc, const char * argv[])
{
    @autoreleasepool
    {
        if (!GetArgs(argc, argv))
        {
            PrintUsage();
            return 0;
        }

        NSDictionary *gifFileProps = @{ @"{GIF}" : @{ @"HasGlobalColorMap" : @YES,
                                                      @"LoopCount" : @(0) } };
        NSURL *outputURL = [[NSURL alloc] initFileURLWithPath:outFilePath isDirectory:NO];
        CGImageDestinationRef imageDest;
        imageDest = CGImageDestinationCreateWithURL((__bridge CFURLRef)outputURL,
                                        CFSTR("com.compuserve.gif"),
                                        [inputImageList count],
                                        (__bridge CFDictionaryRef)gifFileProps);
        if (!imageDest)
        {
            printf("Couldn't create image destination\n");
            return 0;
        }
        
        CGImageDestinationSetProperties(imageDest,
                                        (__bridge CFDictionaryRef)gifFileProps);
        
        for (NSString *inputFile in inputImageList)
        {
            NSString *expandedString = [inputFile stringByExpandingTildeInPath];
            NSURL *sourceFile = [[NSURL alloc] initFileURLWithPath:expandedString
                                                       isDirectory:NO];
            CGImageSourceRef imageSource;
            imageSource = CGImageSourceCreateWithURL((__bridge CFURLRef)sourceFile,
                                                     NULL);
            if (!imageSource)
            {
                printf("Invalid source file path\n");
                return 0;
            }

            if (CGImageSourceGetCount(imageSource))
            {
                CGImageRef theImage;
                theImage = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
                if (theImage)
                {
                    NSMutableDictionary *gifFrameProps;
                    gifFrameProps = [[NSMutableDictionary alloc] initWithCapacity:0];
                    if (gotDelayTime)
                        [gifFrameProps setObject:@(delayTime) forKey:@"DelayTime"];
//                    if (gotUnclampedDelayTime)
//                        [gifFrameProps setObject:@(unclampedDelayTime)
//                                                forKey:@"UnclampedDelayTime"];
                    NSDictionary *frameProps = @{ @"{GIF}" : gifFrameProps };
                    CGImageDestinationAddImage(imageDest, theImage,
                                            (__bridge CFDictionaryRef)frameProps);
                    CFRelease(theImage);
                }
            }
            CFRelease(imageSource);
        }
        CGImageDestinationFinalize(imageDest);
        CFRelease(imageDest);
    }
    return 0;
}