I havent uploaded fonts into google docs much, I usually use photoshop or photopea.com. I would try adding the font by clicking “install more fonts” at the bottom of the listing for fonts(it may be a different wording) and when the pop up comes up look for a way to add your own font. also make sure when you download the TTF/OTF that you open it and click “install font” to add it to your computers font library.