Gypsum fiber shower area

Evening,

I still need to seal my shower area.
What does the layer structure look like (connections in/out of the wall such as water mixer and shower hose excluded)?
-Gypsum fiberboard
-Primer
-Lastogum (gray)
-Lastogum (white)
-Flexible adhesive/tile adhesive
-Tile
-Grout

Can someone tell me how to achieve a watertight transition from gypsum fiberboard to brick/plaster wall? There is sealing tape, but does it also hold on plaster/brick walls?

My tiler (DIYer, so not a trained professional) would not apply special sealing on the brick wall. He is not aware of any problems ever occurring when this is not done. On the gypsum boards, however, it is logical that it must be done.

Would you fully seal the entire bathroom?? Opinions seem to differ here. Some advise me to seal it tightly, while others advise me to keep the walls breathable where possible.
What are your experiences? I tend to lean toward the breathable option.
The bathroom has an exhaust fan and window, so it can and will be ventilated generously.