is your phone rooted?
edit: anything rooted always check xda....but i did it for you:
https://forum.xda-developers.com/mate-20-pro
the link is just for the overall forum. look into the sub forums for gapps if you need it. not sure if it is listed there or not.
just curious why do you need gapps? it should come with most roms. or at least the rom page should have a link for it.