For LRGB with fast scopes and under Bortle 8/9 skies, definitely gain 0.
For narrowband I would use unity. That should be plenty.
The filter issues you have with the QHY163M you will also have with the ASI1600MM. To understand what impacts filter size, use this calculator. Note that the distance of sensor to filter makes a big difference.
Ok, so i will finish those RGB channels tonight with Gain 0, then if i add NB say only Ha i will use unity gain.
Well, i know what do you mean, but i already have 1.25" filters, i can't sell or replace them, and i have to use them, so i have no choice here, also from what i read i heard that QHY163M sensor is about 1-2mm further deeper than ASI160mm one, and although there will be vignetting anyway then i think i can accept the vignetting from ASI1600 than from QHY163M which is more inside, and ZWO has like good way to get the camera sensor closer to the filters, unless you have another solution for me which doesn't include buying another size filters or replace those 1.25" i have.