Did you ever hear about a supermarket and a gas station that closes after 6:30 PM? Well now you have.
I am canceling my Costco card tomorrow (mainly because their car insurances discriminate against European immigrants, but also because all too many times I have been driving there, having forgotten about their funny opening hours). It is not like they need to stop selling gas in the afternoon in order to save money and be able to offer low prices. Why do they have to have a gas employee at all?
They are trying to create a customer hype by all these nerdy lifestyle quirks (show your card to be allowed inside, funny checkouts, unisex bathrooms etc) which will snare customers to buy big lots even though other stores are often much cheaper (for example I recently bought a new 65 inch TV for less than half of what Costco wanted for the cheapest similar model).
Get a price comparison app for gas instead and you will find that even though Costco are usually among the cheapest, there are almost always another gas station that offers the same low price or slightly lower - and they are open 24/7 and dont require an expensive membership.
Hard to know what the gas quality is but my car had problems which could be related to fuel quality (could be unrelated though).