If you are planning on staying F2P, just try to save up for the 10+1 summon, so you have better chances of getting good rewards (Legendaries). Gems being relatively slow to comeby, should be used in an efficient manner. The other one worth using gems on is probably gold, but only when it's has the bonus gold event up. In the beginning it doesn't seem like gold is an issue, but once you start having characters past level 11, each level up is over 2k-5k gold and up.