I believe Team Fortress 2 does something similar, selling hats and such.
that's over the line IMO.
I'd say it's the opposite of over the line. It's a very useful purchase which doesn't have any big game changing consequences. It's just a replacement for party members. It facilitates solo play for people who enjoy it. And the gem store isn't the only way of acquiring this item.
Here's a thread of players complaining about how weak it is: https://forum-en.guildwars2.com/forum/game/gw2/Revive-Orb-Is...
Tangent: one of the best parts of GW2's paid model is that if you stop playing you can get a full refund including what you spent in the cash shop.