"There are a big gate and a small gate at this school."
Yeah, but not everybody uses mobile version. I try to type it on a computer and really want to know why cannot it be 「学校では」
It seems very logical to me, to translate the sentence the way princeorion did.
Reason "There is no で in the selection" does not teach me anything, I learn nothing from that explanation.
Again, this is because, for location, you use に with あります rather than で.
Let's take a look at using different particles with あります and how it affects the meaning of the sentence:
Duolingo's sentence looks something like this:
school は gate が あります ーー As for the school (emphasis), it has a gate.
If we introduce に, the sentence changes meaning slightly:
school には gate が あります ーー In/at the school (emphasis), there is a gate.
school に gate が あります ーー There is a gate in/at the school.
Perhaps the use of に or には may imply that the gate is physically part of or inside of the school, while the use of only は is more flexible, and only implies that there is a gate belonging to the school.