Why is it called a gel? It looks more like a cream.

It is a creme. I dont know why they would say gel.

do you have to keep using this after your pimples are gone just it case they won’t come back?

I personally don't. Just use as a spot treatment. After acne went away completely i do a weekly wipe with benzoyl peroxide face pads

How long will this last if applied over the face once a day?

It's very strong, so you don't need much, like dot and spread a pea size amount, probably less. I use it for spot treatment, and even if I use it every day, mine always last a long time, six months or more. I think if people are using it up within three weeks, they are likely drying out their skin too much. It is a spot treatment, not for your whole face.

Can this be put under makeup?

If you don't glob it on heavily. I apply a thin layer diluted with a bit of water twice a day, followed by a moisturizer to prevent dryness (this is fairly potent stuff and can dry out your skin quickly if you're not careful, even if you have very oily skin like I do). I wait a few minutes to let this all absorb into my skin then apply make-up normally.