As a bonus action, you can expend one use of your Bardic Inspiration to grant yourself a wondrous appearance. When you do so, choose a number of creatures you can see and who can see you within 60 feet of you, up to a number equal to your Charisma modifier (minimum of one). Each of them gains 5 temporary hit points. When a creature gains these temporary hit points, it can immediately use its reaction to move up to its speed, without provoking opportunity attacks.
If you already have temporary hit points when you would gain these new temporary hit points (and you had more before, thus not gaining any from this ability, because temporary hit points don’t stack), would you still be able to use your reaction to move up to your speed?
My understanding would be that no, you couldn’t because if you could, the ability should have been written like this:
When a creature would gain these temporary hit points, it can use its reaction to move up to its speed, without provoking opportunity attacks.
For example, if you have 15 temporary hit points and a bard uses this ability to give people temporary hit points (let’s say 8). Since you already have more temporary hit points than the ability would give you (and temporary hit points don’t stack). Can you still use your reaction to move to your speed, even though you couldn’t accept those temporary hit points?
That is, does it count as you gaining them anyway (even though your total temporary hit points didn’t change, allowing you to use the reaction)? Or does it count as you not gaining temporary hit points (because you already had more than what the ability could give you at the time, and thus not allowing you to use the reaction to move up to your speed)?