A brief plan of action based on UK/European series, taking no consideration of the substantial costs and other barriers that would need to be overcome.
1) Race license (obviously)
2) Club level
MX-5 championship or similar to start you off in racing
3) Club level
Lotus championship or similar to get some experiance of more powerful mid engined cars
4) Maybe
britcar or somthing?
5)
Porsche Carrera cup or similar
6)
FIA GT4
7)
FIA GT3
Expect this to be very expensive and not to make a living out of it unless you're exceptionally good enough to get large sponsors to pay for your costs.
Firstly a disclaimer, I'm not a racing driver and have no experience, but I have followed several drivers and observed their rise to GT championships before.
I'd say this was probably the most accurate, however I'd throw in a couple more championships.
1 - Karting (This is optional, not all racing drivers do it)
2 - Race License
3 - Club Level motorsport such as BARC, Production BMW's etc.
4 - Slightly higher level club motorsport such as GT Cup.
5 - British GT/Iber GT/French GT, wherever.
6 - FIA GT4
7 - High class one make series such as Ferrari's, Lamborghini Trofeo etc
8 - FIA GT3
However, it must be noted GT3 is not for novices. Sometimes the racing they have is more competitive and more close than the higher class GT racing championships. The cars may well be easier to driver than a GT1 car for example but that means the differences in drivers is very much noticeable.
Personally, I wouldn't wait for GT Academy, if you want to do it put your heart and soul into getting yourself to that level. No offence to Lucas or any of the other Academy winners but essentially they just got lucky, that said Lucas in particular is a good racing driving but as mentioned before did have prior experience. Besides, GT Academy doesn't secure a GT3 drive.