Hi all, I'm looking for some guidance on what level to set the gains to on a stock F450, with Naza V2, GPS, iOSD Mini, H3-3D, Go Pro, ImmersionRC tx and 3S lipo.

I have the quad set to the standard recommended gains, which was fine without the gimbal & FPV, but with these things now added it can't maintain altitude while hovering and needs a bit of throttle to "stay-put".  I'm guessing the gains are now a tad low, but rather than randomly adjusting them all up, I'd like some expert guidance.  Any recommendations?  Thanks!

With that type of weight, I would suggest 4s batteries.

I run my fat 450 @ 135 basic and 90 for ATTI. This will give you a nice steady craft for video.

I have exactly the same setup as you with a 3S 5000mah and thrust boosted props and run it at.
118 118 125 156
115 115