I think the big trick with a GT kid is to follow where they lead you. If he's into trucks and engineering, then go where trucks are. Build stuff with him. Give him lots of construction-type toys. Read books about trucks and engineering. Don't push him, but give him opportunities to pursue his interests. If he puts down the trucks and falls in love with something else, pursue that.
It's pretty much what any parent would do, but more intense.
Don't do flashcards and such unless flashcards happen to delight your child. As Grinity wisely notes, your job is to raise a complete human being, not a little knowledge machine. At this stage, just giving your child opportunities to explore and playing with your child--albeit opportunities and play that might look different from the opportunities and play that other parents do with their kids--is the big thing.
It all boils down to one word, really: follow! (And hold on tight!)
