Zuma

Gap bonuses are awarded after shooting in-between two balls (a gap) and completing a set in the next row. Repeat 100x.

Gaps are noted on the scorecards after each level, so if you're anxious or love adding, you can do the math.

You can track the number of gaps you already have:
On the score screen you get at the end of a level, the number of total gaps is displayed with the increase in gaps from the last level in brackets (just add them together).

If you are as bad as I am at this game and just want to get this as fast as possible I found continually playing level 2-1 and pretty much spamming the fire button as a relatively quick way to get it. Wait for the first oppotunity to make a gap (the bigger the better) with your first shot and then just fire away at the gap - I averaged about 7-8 gaps a game doing this in very fast time. Mixing in a bit of actual aiming doesn't hurt either!