Most likely a laserline 212 or 290i. Both are category 2-1 upgrade alarms which is all the gtv would need as it has the cat 2 immobiliser as standard. Unless the original owner wanted the full cat 1 alarm/immobiliser. The 212 is a modular alarm so the siren is smaller with only a few wires to it and then there would be a module inside the car with the main alarm loom on it. The 290i is a compact alarm so the siren is also the main alarm unit, so if you can find the siren it should have the complete alarm harness coming from it. Just to throw a curveball in, is it deffinatly an alarm because laserline also produce the Laserline 310 which is a remote central locking interface which uses the same remotes so would lock both doors but not a lot else.