the GUI textures folder is actually completely unused in current versions of bedrock edition (except for certain touch controls). most UI textures will be in the regular UI textures folder (without the 'g').
the graphics for containers screens are handled very differently in bedrock edition than in java edition. where java edition mainly uses a singular, large, pre-baked texture; bedrock edition uses multiple smaller ones, which are shared amongst a large portion of the game.
the default texture for the background of the inventory is "dialog_background_opaque" and the default texture for item slots (i believe) is "cell_image". both of these textures are ninesliced, so you'll have to get familiar with the nineslice system. nineslice information is not ignored when not included like on java edition.
if you want to use a large, pre-baked texture like java edition does, this is an option, but it requires messing around with the json for the screen itself, the specifics of which may not be consistent across screens due to different screens' different gameplay needs. the inventory, in particular, is very complicated. i can help with the json if you need it.
