From what I've read, the Gate spell can be cast instantly and lasts up to 1 minute. However, I can't seem to find if there's a limit to the number of times it can be cast. Can it be recast immediately after the previous one has closed? Could multiple gates be cast simultaneously?
It is limited by your spell slots/means of casting
While there is no 'limit' from the spell itself in terms of multiple daily castings, you are limited by how many spell slots you have available to cast the spell.
There are other ways you can cast, such as with spell scrolls, a Cleric's Divine Intervention, but as long as you can/are able to find a way to cast it, then you can cast it.
One gate a time
Gate still requires concentration, so unless you have a means to avoid that problem, then you personally can only maintain a single gate at a time.