Found a bunch of functions around the web and stuffed them into a little thing that does operations on GZip files.

Archive.cpp

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <zlib.h>
#include <zconf.h>
using namespace std;

void compress(string filename, string data)
{
    const char * dat = data.c_str();
    gzFile * f = (gzFile *) gzopen(filename.c_str(), "wb");
    gzwrite(f, dat, strlen(dat));
    gzclose(f);
}

bool inflateGzip(string &compressedBytes, string &uncompressedBytes)
{
    if ( compressedBytes.size() == 0 ) {
        uncompressedBytes = compressedBytes ;
        return true ;
    }
    uncompressedBytes.clear() ;
    unsigned full_length = compressedBytes.size() ;
    unsigned half_length = compressedBytes.size() / 2;
    unsigned uncompLength = full_length ;
    char* uncomp = (char*) calloc( sizeof(char), uncompLength );
    z_stream strm;
    strm.next_in = (Bytef *) compressedBytes.c_str();
    strm.avail_in = compressedBytes.size() ;
    strm.total_out = 0;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    bool done = false ;
    if (inflateInit2(&strm, (16+MAX_WBITS)) != Z_OK) {
        free( uncomp );
        return false;
    }
    while (!done) {
        // If our output buffer is too small
        if (strm.total_out >= uncompLength ) {
            // Increase size of output buffer
            char* uncomp2 = (char*) calloc( sizeof(char), uncompLength + half_length );
            memcpy( uncomp2, uncomp, uncompLength );
            uncompLength += half_length ;
            free( uncomp );
            uncomp = uncomp2 ;
        }
        strm.next_out = (Bytef *) (uncomp + strm.total_out);
        strm.avail_out = uncompLength - strm.total_out;
        // Inflate another chunk.
        int err = inflate (&strm, Z_SYNC_FLUSH);
        if (err == Z_STREAM_END) done = true;
        else if (err != Z_OK) {
            break;
        }
    }
    if (inflateEnd (&strm) != Z_OK) {
        free( uncomp );
        return false;
    }
    for ( size_t i=0; i<strm.total_out; ++i ) {
        uncompressedBytes += uncomp[ i ];
    }
    free( uncomp );
    return true ; 
}

bool getGzipBinary(const string& filename, string& contents) {
    // Open the gzip file in binary mode
    FILE* f = fopen( filename.c_str(), "rb" );
    if ( f == NULL )
        return false ;
    // Clear existing bytes in output vector
    contents.clear();
    // Read all the bytes in the file
    int c = fgetc( f );
    while ( c != EOF ) {
        contents += (char) c ;
        c = fgetc( f );
    }
    fclose (f);
    return true ;
}

int main()
{
    compress("archive.gz", "Hello world!");

    string binaryData;
    if (!getGzipBinary("archive.gz", binaryData)) {
        printf("Cannot get GZip binary data!\n");
        exit(1);
    }

    string data;
    if (!inflateGzip(binaryData, data)) {
        printf("Cannot decompress GZip binary data!");
        exit(1);
    }

    printf("Got data: ");
    for (size_t i = 0; i < data.size(); ++i) {
        printf("%c", data[i]);
    }
    printf("\n");

    return 0;
}