how to remove gypsum board cemented to brick wall?
Unfortunately it won't be mortar but drywall adhesive; the so called 'dot and dab' in the UK, and it can be the very devils underpants to remove. It's not particularly hard but it is very 'grabby' and absorbs impacts, which means it sticks really, really well. 

I use a small electrical breaker with a blade chisel. Work at a really shallow angle to the wall and tackle each dot (or dab lol) at a time. The shallow angle will protect the block-work and in theory get behind each clump of adhesive. They rarely come off cleanly (like mortar might come off a brick for example) as it's too soft.  

Expect a solid band of adhesive at the top and bottom.