How do I create this gloss effect?

Hi there,

Currently I'm building a master based on someone else's work.
I'm trying to remake a gloss effect, but I can't figure out how she did it.
In one of the files, I found out how it works.
The gloss effect is a pentagon placed over an image, like in the picture I included.

Can anyone help me on how to recreate a pentagon like this?

Thanks a lot in advance!