That's got to be hard for dad. Counting is not effective at anything other than teaching children how to count or reinforcing the child's knowledge that the parent isn't going to do anything.
This may be as simple as mean what you say and say what you mean. Let dad handle things that go on between him and your son. When dad tells him to do something then he should follow through right away, no counting. Say dad tells Gabe to come to the table. If Gabe doesn't come, dad goes and picks him up and takes him to the table. It sounds like Gabe will respond well, he has been smart enough to figure out already that dad isn't effective.
Another thing that may work well for dad that is very easy is first, then. First we will (whatever dad wants him to help with), then we will (something that will appeal to Gabe). First pick up the toys, then we will go outside.
: Grandmother , 3 Adult Sons