I got the owl mage and the tree. IMO if you really, really want the prize, it's worth it to spend up to a few hundred gems early on in an event. If your account happens to be "lucky" for an event, you don't need them, but if you happen to randomly get "unlucky" drops it's difficult to finish.
I find that spending gems early on helps tremendously, it's worth a few bucks to avoid disappointment for prizes I really really want. We have to remember this is a business, and if playing 100% for free, managing to finish the events should be a welcome surprise, not an expectation, IMO. If this forum were about people playing a game we all paid a monthly fee to play, I would feel differently about people not consistently being able to finish events, because we *would* be paying money to enjoy the game benefits. So if you happen to be a 100% free player, please take this into consideration. FWIW I spent more early on, I spend less nowadays. And that is only because of the gem traps, I routinely lose gems inadvertently so not willing to keep a stockpile anymore.