I´m supprised no one has answered this one yet, so I´ll give it a shot.
FI for the GT is easy... sort of. You have a couple of options. First, the FI from the ´75 Manta, etc is a bolt on to the engine. Then all you have to do is get the appropriate fuel pump, pressure regulator, and wiring. This has been done many times. Just be sure to get EVERYTHING including the computer, injectors, mas-airflow sensor, and harness.
Second, instead of "raping" a Manta (I love them too), you can use a 2.0L FI system that was never imported into the U.S. This is also a "bolt on" for the 1.9L. The OGTS used to keep these in stock, and they will send you everything you need for the conversion.
The third option is to go with a "Haltech" aftermarket computer for FI and ignition. This is the most dificult, and most expen$ive, but gives the best performance. The only parts you need that are not available from Haltech you will need is a Manta/ 2.0L FI intake manifold, injectors (RC Injector recommended) and a throttle body (You could use the Manta/2.0L throttle body, but other$ are better).
If you want to go the Haltech route, here are some FI links:
Haltech - http:www.haltech.com - In my opinion, these are the best "bang for the buck" ECUs available unless you are going to all out racing.
RC Engineering - http:www.rceng.com - These folks know more about injectors than Rally Bob knows about Opels... well since that´s not possible, let´s say that RC Engineering knows their stuff... Hi Bob
My two cents worth - http://www.tgsi.com/fi_univ.html - If you have not messed with FI before, this should get you going a little. As usual, it´s a work in progress... or lack of progress.
You mention you have had carb problems. Before you head down the fuel injection path, have you still got a Solex carb, or has a Weber been installed? FI has some advantages (although some will argue even that piont, especially for upper end racing applications), but is a quite expensive undertaking in comparison to a bolt-on Weber. What carb problem are you having, and what have you tried to date?