gl.glPixelStorei(GL20.GL_PACK_ALIGNMENT, 1);
    final Pixmap pixmap = new Pixmap(width, height, Pixmap.Format.RGBA8888);
    ByteBuffer pixels = pixmap.getPixels();
    byte[] bytes = new byte[pixels.remaining()];
    gl.glReadPixels(0, 0, width, height, GL20.GL_RGBA, GL20.GL_UNSIGNED_BYTE, pixels);
    pixels.get(bytes);

    int bytesSize = bytes.length;
    int[] arrayWitoutAlpha = new int[bytesSize/4];
    int arrayPos = 0;
    for(int i = 0; i < bytesSize; i+=4) {
        arrayWitoutAlpha[arrayPos] = ((bytes[i]&0x0ff) << 16) | ((bytes[i+1]&0x0ff) << 8) | (bytes[i+2]&0x0ff);
        arrayPos++;
    }
    return arrayWitoutAlpha;

GifEncoder encoder = new GifEncoder(Gdx.files.external("testgif.gif").write(false),
                gifWidth, gifHeight, 1);
        ImageOptions options = new ImageOptions();
        for (int i = 0; i < framesArray.length; i++) { //framesArray - two-dimentional int array with frames
            encoder.addImage(framesArray[i], gifWidth, options);
        }
        encoder.finishEncoding();

Pixmap pixmap = ...

int width = pixmap.getWidth(), height = pixmap.getHeight();
Pixmap flipped = new Pixmap(width, height, pixmap.getFormat());

for (int x = 0; x < width; ++x)
    for (int y = 0; y < height; ++y)
        flipped.drawPixel(x, y, pixmap.getPixel(x, height - y - 1));

pixmap.dispose();
pixmap = flipped;

int pixel = pixmap.getPixel(x, y); // RGBA

int r = (pixel >> 24) & 0xFF;
int g = (pixel >> 16) & 0xFF;
int b = (pixel >> 8) & 0xFF;
int a = pixel & 0xFF;

pixel = (a << 24) + (r << 16) + (g << 8) + b; // ARGB

Pixmap pixmap = new Pixmap(width, height, Pixmap.Format.RGBA8888);
ByteBuffer pixels = pixmap.getPixels();
gl.glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
frames.add(pixmap);

// Note that if loopCount is less than 1, then the gif will loop indefinitely
GifEncoder encoder = new GifEncoder(..., width, height, 0);

ImageOptions options = new ImageOptions();

// Frame rate / Time between each frame
options.setDelay(50, TimeUnit.MILLISECONDS);

for (Pixmap pixmap : frames) {
    int width = pixmap.getWidth(), height = pixmap.getHeight();
    int[][] pixels = new int[height][width];

    ... flip pixmap on the y-axis ...

    for (int x = 0; x < width; ++x) {
        for (int y = 0; y < height; ++y) {
            int pixel = pixmap.getPixel(x, y);

            ... change pixel format ...

            pixels[y][x] = pixel;
        }
    }

    encoder.addImage(pixels, options);
}

encoder.finishEncoding();

new Thread(new Runnable() {
    @Override
    public void run() {
        ... save gif ...
    }
}).start();

for (Pixmap pixmap : frames)
    pixmap.dispose();
frames.clear();

PixmapIO.writePNG(fileHandle, pixmap);

import static com.badlogic.gdx.graphics.GL20.*;