Salarywise, we could get Gay in a trade for Sheed and Perk, and maybe Davis if Gay's demands were high enough - and I do think he's looking for a deal starting above $12.7 which is the max the Celtics can take back for a Sheed/Perk combo. Then, though, there's really not much of a way to get real big men in here.
Or we could get him in a trade if Pierce opts out and we renounce our rights to all our free agents, and send them just Wallace and maybe some sweetener like a future 1st. Then our roster would be Rondo, Bradley, Gay, Garnett, Perk and Davis, with only minimum contracts available to bring in extra players. Rondo, Bradley, Garnett, Park and Davis counts for about $41-42 mil against the cap, which is I think $56.1 this year; we also have to hold enough rookie minimum salaries to fill out the 13 man roster. Those holds cut our available cap room by another $3.3 mil or so, so Gay would have to agree to a starting salary of $10.8 mil - $11.8 mil.
We could also get him if we send out Garnett to some team (can't imagine Memphis would want him), or if we signed and traded Pierce or Ray or some other combo of guys.