What's speed gain and how does frame rate setting determine screenshot frequency?

Frame rate defines the (planned) video frame rate. So lets say 25 frames/second and game speed it 60 ticks/second, then you will take every 60/25=2.4 ticks a screenshot.
But the video will be 'real time', so frame rate compensates game speed to video speed! (to keep the speed gain more understandable)

Speed gain can be used to speed up the time lapse effect of your video, so a gain of 2 will speed up the time lapse by two, using earlier settings you'd take on screenshot per 2.4*2 = 4.8 ticks.

So if you know how long your game is (likely) to take and you know how long your video should/could take you can use speed gain to make this happen. Let's says game takes 100 hours (or 6000 minutes) and video 10 minutes you'd need a speed gain of 6000/10 = 600.
But of course you can keep it simple and think that a certain speed gain just 'looks nice'.

As you can see both frame rate and speed gain both determine how much ticks will pass before a screenshot is taken. Therefore, if you 'mess up' one you could compensate changing the other when editing the video. (within limits of course)

Thanks, I understand now.

This would be nice to include in Information or FAQ section. Right now it's easy to find with only 7 discussion threads, but in time it might get buried in other pages.