A quick demo of using fill_gradient per a request on the FastLED Google+ community.

fill_gradient_demo.ino

#include "FastLED.h"

// fast led constants
#define DATA_PIN    3        // change to your data pin
#define COLOR_ORDER GRB      // if colors are mismatched; change this
#define NUM_LEDS    28       // change to the number of LEDs in your strip

// change WS2812B to match your type of LED, if different
// list of supported types is here:
// https://github.com/FastLED/FastLED/wiki/Overview
#define LED_TYPE    WS2812B

// this creates an LED array to hold the values for each led in your strip
CRGB leds[NUM_LEDS];

// fill_gradient variables (except direction as I wasn't sure what variable type to make it?)
int start_pixel = 0;
int end_pixel = 28;
CHSV start_hue = CHSV(0, 255, 100);
CHSV end_hue = CHSV(180, 255, 100);

void setup()
{

  // the wiki features a much more basic setup line:
  FastLED.addLeds<LED_TYPE, DATA_PIN, COLOR_ORDER>(leds, NUM_LEDS);

}


 void loop() 
{

/*
  arguments are:
  array: the name of your "CRGB name[NUM_LEDS] definition above
  start_led: which index number (0 is first pixel) to start with?
  start_hue: CHSV form (hue, sat, bright) of color/brightness for start led
  end_led: which is the last pixel in the gradient?
  end_hue: CHSF form for color of last pixel in gradient
  direction: thinking of a 0-255 color wheel, which direction should the
    gradient ramp through? Change the below to SHORTEST_HUES and you'll
    see that it goes red to blue through violet. Compare that to this, which
    goes the "long way" through yellow, green, and cyan.
    directions explained here: http://fastled.io/docs/3.1/colorutils_8h_source.html
*/

  fill_gradient(leds,
                start_pixel,
                start_hue,
                end_pixel,
                end_hue,
                LONGEST_HUES);

  FastLED.show();

  delay(1000);

}