On the 109 G-6, the gondolas increase weight about 500 lbs. I don't remember the exact top speed reduction, but the average rate of climb is reduced by 10% (taking off with 100% fuel). Handling & turn radius suffers as well, but I never tested this aspect thoroughly.
Personally, if I see the need for massive firepower, I take another plane anyway (109K, 190A8).
The only time I do select the gondola option on the G-6 is when intercepting bufff in MW arena.
I never take gondolas on the G-2, for I don't see any reason to downgrade the very characteristics that made me pick this plane in the first place.