Gondolas are mission-specific.... If you're going up against fighters, if you're escort, sweep, CAP, whatever... leave them off.
If you're intercepting bombers, or hunting really heavily armored craft (IL2, for example) you may need the firepower.
I made a chart for TracerX once, when the question of gondola performance loss was called up.
Not only does it mess up your level top speed, your roll rate, your turn rate, and your ability to dogfight, they screw up your ability to climb, stay in formation, etc.
A 109G2 without gondolas will climb better OFF wep than a 109G2 *with* gondolas will climb WITH wep... It's a big difference.
Assuming the drag is the same for all 109 gondolas (it should be) you see how it can really impact your game. So depending on the mission it may be important, but your "firing time" does not increase any longer with the extra ammo. I've landed 9 kills or so in a G6 with 1 hotpad halfway through, no gondolas. If you practice your aim that single 20mm nose gun can definitely do some damage.
EDIT: Since the chart was intended for FSO use, I don't recall if it was with DT or without... I think "without"...