Gill put his side ahead midway through the second half when he volleyed home from a tight angle in front of a bumper crowd of more than 8,000.
Exeter survived a late scare when Wycombe rallied, but they were good value for maximum points in a tight encounter.
Wycombe's John Akinde forced Paul Jones to make a fine double save with a shot from inside the box in the first minute.
On the half-hour mark a long kick from Paul Jones found Stuart Fleetwood and the striker darted forward before seeing his 12-yard strike saved by Jamie Young.
Chris Zebroski then forced Paul Jones to make a good save from his fine header following Matt Phillips' cross.
City boss Paul Tisdale swapped strikers Craig McAllister and Richard Logan for Steve Basham and Liam Sercombe in a bold move at half-time.
Four minutes after the break Ryan Harley put Fleetwood through and the striker's shot grazed the crossbar.
Exeter were the better side after the break. They were ahead when Harley's fine ball into the box was flicked on by Logan and Gill was in the right place to volley home from a tight angle.
The Grecians could easily have stretched their lead. Young saved superbly to deny both Harley and Rob Edwards and McAllister headed the ball against the bar from Moxey's corner as Exeter dominated.
Wycombe still posed a threat and substitute Jon-Paul Pittman forced Paul Jones to save well with a strike from 15 yards.
The visitors threw everything at Exeter in the closing stages but the Grecians held on for a crucial win.