The GMF lease assumption process is garbage.
Once you do all the paperwork, and GM accepts it, they will tell you that the truck is no longer yours and you shouldn’t drive it.
HOWEVER
They will tell the “buyer” that the truck is theirs but not to drive it until the paperwork is complete. This puts everyone into a weird grey area situation. The “buyer” is definitely going to drive it, and is going to be pretty irate about the whole situation and take it out on you when it takes GM 4-6 weeks to deliver a half dozen sheets of paper to you/him.
Find a friend/relative/someone you know to transfer it to.
Source: Did a GMF transaction earlier this year. Almost went nuts on the seller. Definitely went nuts on GM Financial. Took 4+ weeks longer than it should have. Private Transfers: What's the paperwork holdup from the finance companies?!