Hence my comment above that if you can get someone to make such a small fusion engine, you can put a PPC on it; the stumbling block is that small engine. See the Savannah Master fluff for details. The Omni 25 doesn't re-enter production until years after the Helm Core is disseminated. And I can't find anything that takes a 50-rated SFE in the Succession Wars time period, suggesting that no-one has a line set up for them.
A fusion-powered Warrior with a PPC or LL qualifies for a gunship in my book; but canon argues strongly against the necessary engine being available in the time period.
Yeah, exactly what you're saying, *if* you could get the fusion engines, you could make some pretty good VTOLs with base tech (though I'm actually of the opinion that Warriors are pretty decent gunships in the era, especially the 7C and 7A but that's off the point I suppose). The problem is justifying where the engines come from, at least if you want to follow canon strictly. Of course for an individual campaign there are ways to get around it if you're really set on it - i.e. your mercenary unit could always stumble across an old SL era cache with engines, much like the Omni 25 cache that resulted in the Savannah masters.
However, I'd say the reason for the lack of a true Gunship during the 3025 era makes sense in universe. At least one using a SFE, there are certainly some optimizations that could be made even sticking with an ICE engine.
But as far as I'm concerned the real question is why didn't the Star League have better VTOL Gunships (or why don't we know about them)? The Cyrano with a large laser and the Nightshade royal with a large pulse laser are about all I can think of. And both require getting closer than I'd like with a VTOL, as much as I personally enjoy the maligned IS large pulse lasers. The Royal Cyrano with its several improvements, including an ER LL is as good as they get, afaik.