So, I happened to get lucky enough on my final pull with tokens to get Guardian Gazelle. I wanted to get some advice on if Gazelle should jump to the front of the line with regards to my yellow heroes.
Here is my Hero Plan to see where my roster is at.
Out of that list, Gazelle would get my vote.
However, before you ascend to tier 4 - See how your luck goes in the Costume portal with the new Leo (either next week, or ~5 weeks from now depending on release).
Thor and Odin come out in Valhalla soon also.
I tend to agree with Gazelle getting my vote too.
I will hold off though, see if the costume portal is nice to me.
Gazelle all the way. The one you have is always better than the one you might get.
I’d do Norns before Gazelle if I had both 
Gazelle is one of the best attack heroes in the game. She doubles attack, halves damage and self heals. She is also undispellable and removes all buffs/ailments from your other heroes and prevents the other heroes in your team from getting buffs/ailments. She’s not so good in defence as she can be targeted and if she dies you take a hit. But on attack, paired with damage dealers, she’s brilliant. You won’t regret maxing her. She’s particularly good on season 3 hard levels, keeping everyone alive without the need for too many battle items.
Can I ask why Norns? An alliance mate of mine is currently leveling Norns with the intention of putting them on defense. I understand Norn’s effect, but I don’t see it being defense-worthy.