well, i think by definition, a link is when one move combos to another but the first move fully recovered. (so compared to a gatling/special cancel, the first move didn't cancel to the second move.)
so, to combo them, you let the first one recover, then do the second one fast enough that you get the BEAT counter going. The timing depends on which ones you're talking about.
But just so you know, some of them actually ARE gatlings. if you look in the player's guide on this site (check the link in the announcements section at the top of the Foundation board) and read some of the comments in Slayer's movelists, you'll see that it says that some moves have gatling windows at specified times.
in any case, it'll take some practice before you get the timing down consistently for his gatlings and links.