Hi ... Just one Qs.

Good afternoon, classmates! :)

Please, could you help me with this sentence:

“Who said there was someone at the gate? There is not anybody there.”

Is it wrong? Someone told me I can't use "anybody" because is a double negative.

Thanks in advance.

Yes, it is wrong.  It is grammatically incorrect.  It is better to use the following:

"Who said there was someone at the gate?  There isn't anyone there."

"Who said there was someone at the gate?  There is no one there."