Inventory screens and whatnot are no small task in GB Studio and definitely goes outside the scope of what I can provide here.
As a suggestion, in Abducted I simply used a dialog box with inline variables for items. For a new project I’m replacing unused characters from the font file with item icons and using those for key items.
Just note that the graphics for rendering the view uses most of the tiles allocated in GB Studio so you’ll need to account for that if you plan on incorporating such a thing into the background tiles.
Also to answer an earlier question, GB Color mode is not supported by the plugin. You could maybe use a single palette for the view without trouble but that’s only a guess and not tested.