There are some GC specific items so you want those then yeah you'd need to rank up in them and unlock them. Most of the times are just furniture, bardings and minions. There are some equipment that can't be found with a different color in the GC, but it's not going to break you. When you hit max rank, there is some decent 40-50 gear, but there is also the first easy to get weapon when you hit 50, is the GC weapon which is item level 55. So most grab that if you are just starting and you need max rank for it, which sounds like you have.

Beast tribes should be unlocked at this for the first quest so you have access to the materials they sell. If you ever plan to do any crafting, these vendors from the beastmen can be very help if you don't want to spend the time crafting the items and just want to buy them. Also, if you're a collector type, they all have unique mounts and minions you can't get anywhere else and only unlock at max rank with the beastmen. They are also a good way to get ventures if you don't have the seals to buy ventures.

They also said that there is a separate quest line that will unlock if you have all 5 tribes completed when the Ixal come out in post 2.3.