While I don't have a ton of experience with Gruntz, I have played it in both 15 and 28mm, and I think it works well at the larger scale.
We made a couple of changes at 28mm
1. Range distances and movement rates were increased by 50 percent. So a range of 20 inches was now 30, and a move of 4 inches was considered 6. Played on a 4'x6' board, most units were in shooting distance on turn two, three at the latest.
2. We also built units as "specialists," so every figure had multiple hit points. Figures were then placed into ad hoc squads of three to four figures. We used small dice to mark wounds on individuals as the game progressed.
Another time, I ran a space-hulk-style game using 28mm figures with the same changes above. Along with the infantry, mechwarrior clix miniatures were used as terminator-style battlesuits (built as scout mechs), and that worked well, even in the confined spaces.
Overall, Gruntz seems pretty flexible. My main complaint is that I wish the weapons were more differentiated, but that's a minor thing and has no bearing on scale. :-)
Hope this helps.
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