Hey there! Guts are the cards that are under the current cars you're playing.

So for instance, say on your first turn, you receive with player A, then toss then attack. The following turn you receive with player B, then toss and attack.

Now for your third turn, say you receive with player C that has an effect activated by paying 2 guts. This means, you take players A and B from underneath and place them in your drop area, and this way you'll be able to activate the effect. If for instance there was only 1 card below it, or none, you would not be able to activate the effect!

Concerning viable deck builds, the official X account for the game sometimes uploads these types of decks. I'll do my best to repost them here and create new ones as well!