GigaSampler defaults to midi volume= 90(approx.). Even if you pull the volume fader in GigaSampler to the top, you will only achieve ca 100. To get midi volume=127 you will have to send a volume=127 message from an external midi source, like your sequencer, or keyboard.
The midi output is limited this way to prevent distortion if several instruments are played simultaneously. In GigaStudio you can pull the midi fader all the way up to 127.