if changing a caliper is neccessary to bleed the system after? also would a rear sunny gti caliper be ok ?
Yes you need to bleed, but so long as you clamp the pipe (assuming you have a rubber flexi and not braided) you won't need to bleed much.
I generally clamp, change calliper, undo bleed nipple, un clamp, allow the fluid to run through by itself until fluid comes out of the nipple then tighten nipple. In most cases like that, there's no more air to bleed out.
Haven't a clue about gti calliper.