And I have researched it on the web and found something about some
reaction between cement and gypsum.

Nothing is going to work well if the plaster/plasterboard gets wet. There
are plenty of adhesives that will work on gypsum plaster, but if you get
water in behind the grout, the tiles will ping off and the plaster will
degrade.

I'd probably use something Like Ardu-Flex 6001 or BAL White Star, although
it would still be better to replace the plaster background with something
better.

If you don't replace the plasterboard with something waterproof, ensure you
do the tiling job thoroughly. Use an adhesive suitable for swimming pools
such as those mentioned above. Ensure complete coverage. This is not a time
for dab and dot, you need 100% fill between tile and plaster. Remove an
occasional tile during fitting to ensure that all ridges were compressed
flat.

Pay very careful attention to grouting. Use a flexible grout such as
ArduFlex FL/FS to reduce the chances of cracking. Keep the grout lines
narrow. Seal the grout using Lithofin StainStop (KF or MN, probably KF for
the types likely to be used in a shower). This will also stop the grout
going mouldy and will encourage water to bead and run off, rather than
penetrate the grout. You may wish to apply to the tiles as well as the grout
lines, depending on the level of glaze. Use silicone sealant for internal
corners of walls.

Christian.