Diesels aren't converted .. it is added .. or you change the engine!
GTi's go well on gas. We did a GTi turbo once .. flying machine.
Old Gti's with K Jetronic were also great.
Make sure you have an LPGA certificate with it. I'm a member of LPGA actually .. If you need, I'll elaborate.
Tax .. nothing, as even changing Registration Doc to alternative or duel fuel makes no odds. Insurance may/will need to see copy of the PLGA certificate issued by the converter .. or you can do yourself and get it inspected and certificate issued.
Al.