A gap is a difference between your mastery level and your character level.
The larger the gap, the less xp you get (pretty significant) and the more sp you get (less significant, yet still there).
I seriously think this explains it well, but this site MIGHT help you understand it a bit more: 
http://www.janrique.com/silkroad/SP.htm
As you can see, the larger the gap, the more sp you make.
There was this one discussion which told the numbers, I can only remember two of them:
Gap 0: X1 sp.
Gap 9: X19 sp.
X being times, thus at a 9 gap you make 19 times more sp... But it wasn't really times, it was some calculation... Anyways, I'm probably just confusing you. So... Bigger gap: More sp per level.
[EDIT:] cin explained alot of stuff I didn't. His explanation is better, but mine might help a bit too, so I'll leave it here.