Have you done any research, like contacting gas safe or a training centre, into what is required Steve or did you just think I'll asked on hear to save the time ?
Go on then I will bite.
What makes you thing you could become a GSR Engineer then ?
I take it you do not have an NQV in plumbing or heating, so you will be a category 3 entrant therefore you will need complete around 20 - 30 training days at a centre doing the theory & you will need to work along side a GSR engineer carrying out a range of gas work, including installation, testing & commissioning on boilers cookers & fires as a minimum the GSR engineer has to sign off your log which must be a minimum of 100 day's (might be more, I will have to check).
Only after completing the on & off site training can you sit the CCN1 test which is the core safety, once you have that you can sit the appliances that you require to work on (boilers, cookers, fires etc etc)
Once you have all those you can then apply to Gas Safe to go onto the register (after paying your fee's) oh & don't forget you will need to do your Part L so you can register each boiler with the council.
Or did you think it was as easy as turn up & pay over some money ?
Stick with the building mate & find a good plumber / heating guy ! (helps if you offer a bit more lolly, you tend to get the better ones interested).