....working with a criminal teleporter, say, Spiral (or, say, Diablo), the GG and Spiral teleport into the Oval Office, the GG taps Bush on the neck from behind, and they teleport out, either with the stone Bush statue, or leaving it behind to be found by the secret service--or perhaps first crushing it into rubble.
They might do this on their own, but more likely, as a huge terrorist attack on the country or the West itself by a new Masters of Evil.
Bingo. The GG has just been 'intelligently used.' ThatGuy speaks as if the GG's limited powers in themselves make him a patsy. Most heroes and criminals have limited powers.
And speaking of Diablo, Kurt made great creative use of him during his Avengers run.
Just because a villain was used in the Silver Age...ah, thinking they're useless is most likely a prejudice of 'those who weren't there then.'
And that does make a difference, because to newer readers who first encounter Diablo in an FF Essentials volume, OF COURSE he seems 'dated' and 'older.' They're coming at him from an entirely different perspective than readers are who were 'there then' and grew with him, the GG, and a zillion other good and bad guys and gals.
I too am tiring from the obligatory "look, I'm an obscure villain, now punch me out" routine that writers employ to make readers think that they don't ignore past continuity. They might as well create a new villain if the only purpose is to get clocked.
Although I'm not saying it's needed, but in this era of secondary mutations and enhanced powers for every other character around, would it not be unreasonable to have the Grey Gargoyle perhaps display some sort of control over granite itself? Maybe use chunks of stone as projectiles? I'm not saying turn him into Terrax, but increasing his skillset, IMO, would only be beneficial to the character.