would be necessary to know if these gmt files come from a simbin games or one that use other versions of the rfactor engine, anyway both wtcced and rfactordec have the same exact syntax so no problem:
all the needed informations and tools you need are located on
http://aluigi.org/papers.htm#wtccedhttp://aluigi.org/papers/wtcced_all.txtthat quick step-by-step should be easy and understandable, if not tell me what you don't understand so I can try to make it more simple (although I don't know how).