Actually, if you're staying "stock", the compressor is THE most common GM (Frigidaire) A-6 type, but the original engine mounting brackets AND the IN/OUT hose adapter that mounts on the back of the compressor are a "BITCH" to find!kwilford said:drummertom,
Welcome to our site. I moved this thread to the "Engine Swap Forum", where the best minds in Opel-dom have discussed this topic extensively.
As for AC in a GT, you either need to find an existing car that is suitable as a donor (fat chance) or find a full stock set second-hand, which includes the condenser, evaporator and fan (which sits in the luggage area), the tunnel console with cooling vents, and a compressor (fatter chance!). There has been some discussion on aftermarket AC kits, but to date, I haven't heard of a single successful install.