In the case of Garruk’s Packleader, you would play all Green or colorless cards. He’s popular in that format because there are plenty of creatures with power 3 or greater at common, especially in green, and card draw is always awesome.
The awesome thing about Pauper EDH is that it opens up possibilities that regular EDH doesn’t offer. Think of all of those cool uncommon creatures that just never served any purpose in Constructed decks. If you can find enough commons that thematically support it in a singleton format, go for it!
One awesome thing about Pauper EDH is that many EDH staples are in fact commons: Command Tower, Darksteel Ingot, the various Signets, and many more. The power level of cards is nowhere near traditional Commander and you don’t end up with a lot of the super expensive lists that you may find in regular Commander.
In the near future I’ll be going over some popular and powerful Pauper Commanders. While you’re at it, though, take a look at your collection and see what Uncommon creatures you may want to build around. Remember that color identity rule is exactly the same in Pauper EDH as it is in regular EDH. This means that abilities that call for different colors of mana than those in the creature's mana cost will you to play cards of those colors in your deck, as well.
For the complete Pauper Commander rules, check out the PDH Homebase!
So who will be your Pauper Commander?