You earn favor which can buy individual gods and some skins I believe. You can buy gems for real cash and use them to buy skins that don't sell for favor as well as voice packs (never messed with those yet).
Favor seems abit slow but doable, I've played a decent amount of time over the weekend and have 9000 favor, i saw some hero's i think for 9000 and some for 15000 so it can stay free to play if you want. Honestly i think its a no brainer to buy all the gods now and in the future (is this limited time?) for 29.99. Seems kind of stupid actually they would have this but hey, whatever works for them!