Jau69
I would expect your GT to have either a 1.9 litre engine or if your after rarity you could be lucky and have a 1.1, although the latter would be able to pull the 'skin off a rice pudding'. Sorry that English saying but basically means it's a slow car.
To tell the difference the 1.9 will be a much larger engine and will feature only a single carb, the rocker cover will also be Aluminum with OPEL embossed on it. The 1.1 engine will have a twin carb is smaller and has a black steel rocker cover.
To find out the displacement of the engine, excluding any possible rebuilds you should check out the engine number which is located on the drivers side of the engine block. The number will be stamped on a flat plate that is about 5cm's to the right of the dizzy. The block will should be stamped 1.9s followed by a series of numbers.
If it's ever been replaced it could be stamped, 1.9, 2.0, 2.2 or 2.4 and this will indicated the engine displacement in litres.
I hope this helps.