I've done something similar.
All you have to do is save the GM_xmlhttpRequest response in a DIV.
With this DIV you can do whatever you want (show, hide, display only some of the content, etc)
Just take a look at my script source code.
I'm positive it will help you.
I know you don't need, Mr. 14k rep, but i'll break it down for you anyway :)
function conectar() calls GM_xmlhttpRequest [GET] and stores only the part of the content that i want to use in #divtempora, which is a dummy div that the user never sees (hidden).
Then function resp_dxlegacy() walks through the dummy div, save the info i want in a variable and calls conectar() again passing this parameter and storing the content in another div, which is, finally, displayed to the user.