Yeah, there is no amount of splitting or automatic decimation that is going to make that into a usable game asset. A game where each player’s portal gun has more triangles than most games’ entire worlds (with all buildings, terrain, and characters combined) is not going to be playable.
The real answer is that you need to learn low-poly modeling techniques for making game assets. It’s very different from making assets for non-realtime rendering. That portal gun shouldn’t be 450,000 triangles, it should be more like 1500 at most. Details like the turbines in the rear: those would normally just be a texture, not geometry.