I'm no hotshot (just check my scores) but I've come to the conclusion that the 109-G2s best attribute is its mix of BnZ and turn-fight capabilities, so long as you stay away from Spitfires!
Use the boost to get to 10 or 15k. The 109s seems to last longer than the boost in other planes, and the motor seems to cool faster, too, so you can boost again when you get ready to engage. Sometimes you can use the motor's power to keep your nose up longer than the other guy, particularly in a turnfight against one of the USA planes.
When it stalls, it stalls off to the right harder. In other planes, it seems to be the other way around. Might be an advantage if you stall to the right and the other fellow on your six stalls off in the other direction.
BnZ is a bit trickier because you have to split-S to lose altitude rather than go straight in like a Mustang. As you've learned, the elevators tend to stiffen up a lot when the speed increases. The G-2 seems to gain speed faster than the F-4, so pay closer attention to your dive if you're in a G-2 (and even more so in the G-6).
For gunnery, the cowl guns and nose cannon allow you to set your convergence out as far as 600 and still hit anything between your nose and 600 out, and not have to worry about keeping your wings aligned with the target's wings.
Stay away from Spitfires, Zeroes, and N1K's, as they can out-turn you and outgun you. If you see a USA plane, go for it! Typhoons, FWs, and LaGGs are also good pickings if you can catch them.
Don't use the gun gondolas - too much weight and drag. The extra gunnery is too high a price. The only reason to carry them is if you plan on doing bomber intercept or ground attack, and if that's the case then you'd probably want the FW or one of the American planes for their higher speed, lethality and ordinance capacity. But, do carry a drop tank with 50% gas in your fuselage tank. Plan on using the drop tank to get to altitude, then drop it when you're ready to engage. Fight from 50% to 25% gas, then look to get out, depending on how far you are from your base. Too many times I've died from fuel starvation, but I've never run out of ammo (well, maybe cannon rounds).