How do we make an item look different in the gui when we hold it? Or vice versa?

MCreator allows you to create animated textures, but the process is a little different from the regular textures; you cannot create the frames in MCreator nor can you import frames from existing textures, you need to create and import the actual image files themselves into the animated texture editor.

You can create an animated texture from the resources tab, click create new texture, then click the 3rd button on the dropdown menu.

Once you're done you save it as an item texture and apply it to your item.
This will also apply to the item in-hand.
If you want both the GUI and in-hand textures to be different... I don't think that's possible. Maybe messing around with custom models could be of some use?? But I don't think it would work well at all, and most likely not in the way you want it.