It would be much like when D'Lo returned with a small 'we missed you' cheer, but half the crowd will still not remember him and then, like D'Lo, the crowd will not give a damn again. The audience likes legends, not failed middle carders from the past. At half way to 40 and about 4 years since he's been in the WWE, I don't see a return to the WWE in the books for Gunn. He's well past his prime and offers nothing to the WWE. A D-X reunion is long over, bring them back again with the middle card rejects Dogg, X-Pac and Gunn, just wouldn't suit well with the current HBK and HHH, especially with HHH now officially back to being married to Steph in storyline universe again. Might've worked when D-X first got back in 06, but not anymore.
WWE gave Gunn many chances to be a star and he dropped the ball every time. The crowd liked him, but much like many of the middle carders of the attitude era, never got behind him enough to put him in the main event. He was an average worker and wasn't terrific on the mic, no better than a Venis or a D'Lo and well below Goldust. When Gunn leaves TNA, I'm sure his wrestling career will be pretty much over, save maybe some independent appearances if he still wants to wrestle. So to me he should ride out as much as he can in TNA and retire into the sunset without much fuss.
Best case scenario, he may MAY be a HOF (hey with KoKo getting in, Gunn's accomplishments are boarderline, and he was a member of 3 successful tag teams) it's definitely a long shot and I don't support Gunn getting in, but it's the best and should be the last and only thing we ever does with the WWE again, if it were ever to happen.