From the photos, the generic AT case seems to be in decent condition. Is that a CMOS battery pack attached sitting in the bottom of the case? I can't see the entire motherboard but it looks like has no on-board battery, and that is a major plus as it looks like there has been no battery damage to the motherboard. I also see the words "California USA". I'd try and get the part number for this board if it has one printed on it. Is that a 486 board?
The video card is an ET4000 based Diamond Speedstar ISA card. That is a good card.
As I mentioned these clones ranged from budget to gaming, and I'd call this one more of a "business" class, somewhere in between.
It would be best to try to sell this one whole. I think there would be a good amount of interest in this, specifically because of the case.
Because these literally have no name it is hard to search eBay, but glancing at eBay sold prices for similar items seem to be all over the place ranging around $50 to $200. Given the pluses, perhaps ~$150+SH? Perhaps a bit higher if it cleans up really well and is shown to work.
You did not show pictures of the back of the Gateways case, but those cards look like they have some corrosion on them. I don't see any sold towers but Gateway 2000 desktops seem to also sell around $150, sometimes more if they are in really good condition. Whole would be better, but if there is a lot of visible external corrosion, it might need to be broken down.
If you list them on eBay, you probably want to decide how much you want for them and list as "buy it now".