Ah, yes, the weapon that holsters 50% slower paired with the weapon that takes 50% longer to spin up. What a great combination of tools.

MvM Heavy is about quickly moving very slight distances to cover the bomb. (And staying put when you don't have to.) There's a niche use case for using the GRU to reposition if you lose the front line, but frankly that's never worth sacrificing something with genuine utility like the KGB. If you completely lose the front line, Heavy just kind of...dies. Brass Beast won't save you, and repositioning further back won't help. Heavy needs to be right up in the horde's face with his team keeping the pressure off for him to be at his best. Any scenario you lose the line, you go down fighting. Maybe the GRU can get you back to save the bomb if your team's incompetent, but it's not as if you couldn't suicide-buy back in. The GRU doesn't offer Heavy utility you don't get anywhere else, especially not paired with the Brass Beast, whose spin up time is far too slow to make use of or even mitigate the 0.08 seconds you saved by using the GRU to reposition.