Ok, what i want to do is make it so that every 10 kills you get 20 gas and the kills are reset to 0.

Here's what i have:

Condition:
Player 1 kills score is at least 10

Actions:
Modify resources for player 1: Add 20 gas
Modify kills score for player 1: Subtract 10 kills
Preserve trigger

What this does is it gives me 20 gas per kill instead

kill score is not kills. Here are some kills to cash systems, with more map detail we could help you choose the most suitable: http://www.maplantis.org/index.php?pg=wiki;id=45