its pretty simple mt freind, gap is so you can get more sp with the sacrafice of exp. you can heve a gap from 1 to 9. your gap is determined by uor lvl and your highest mastery lvl. so take tis for example, your lvl 20 and your highest mastery lvl is 17, so you have a gap of 3. oh and as a rouge start sp farming a bit now cause yuoll need it, just keep a gap of about 3 or 5.
Yeap that's sound nice to up every skills at 0 gap isn't it? But when you reach higher level, you will probably to have your lvl gapping between your mastery and character level, simply because one skill needed a lot of SP in order to upgrade to the next level and you can't do it in 0 gap, that's why SP farming or level gapping comes in place. For example you need to up that particular skill for an amount of 1k SP, and with that of course you cannot do it in 1 level, you see. So that's why needed a gap between your masteries and char level so that you can get more SP.