Can someone explain me how fil type set to camera film gate in projection node works?

I know how fil type set to camera resolution works but not quite about camera film gate.

When i set fil type to camera film gate then can i use different image resolution than image resolution in render settings? i.e. 2000x1000 and 2543x1280?

And where i can get information about camera aperture when i dont know nothing about camera whitch the picture was taken?
Do i have to use only pixel aspect ratio of the image (i.e. when i creating mattepainting from scratch) or there is a way to calculate camera aperture correctly?

Will this projected images be correct when i render whole scene?