Petty is a more than capable defender, and with the likes of May and Lever, they form a very good defensive unit.
Look what happened when Trac played more minutes forward, it freed him up and he broke the shackles. The same would happen with Gawn. Play Jacko #1 ruck for a while (if he kills it, it will only drive his trade price up if he goes), plus Gawn as relieving ruck is a daunting prospect, and would take heat off Brown in the forward line.
Petty stays back