I have not done the Gustav so this is pretty general. You need to remove the caliper and pop the pistons out. If this is a monobloc caliper(which it looks like from the one picture I saw on their site) I would do it this way: I use air pressure(Use eye protection, point the caliper so the piston will not hit you) You will probably need to hold one piston down in the bore while you pop the other out. To get the second one out, block the port of the removed piston and apply pressure again. If you can soplit the caliper do so and remove the pistons the same way, don't lose the little oring between the two halves. Once the pistons are out remove the seals and clean them with the correct mineral oil. Clean the bores with rubbing alcohol. Lube it up with mineral oil and reassemble. Then hook the caliper up and bleed out tthe system completely actioning the lever while doing it.
An easier method you could try is bleed the system out til you get fresh oil, then with the pads out(You should take em out to bleed anyway)using a wooden dowel or some other kinda soft stick like object, push the pistons back in their bore, then press the lever to expand the pistons, then cycle them back in with the stick. Do it a bunch, bleed to fresh one more time and you should be good to go.