The drain in my garage was rusting so I repaired it. I’m having the epoxy flooring done tomorrow and painted the walls to get ready for that. Now about three weeks later I have a sewer gas rotten egg smell in the garage. The drain still has water in it and looks fine. I even bought a 4 inch cap and put on it and it still smells. I was up on the roof to make sure a vent wasn’t plugged which it wasn’t. I am at a loss as to where this smell is coming from! Any ideas?

peppermint oil test is the next step.
couple ounces of peppermint oil down the vent stack followed by some very hot water. a gallon or two should be plenty.
don't go in the house because you will have the smell of peppermint on your clothes.
have someone sniff around to see if the problem area can be located.

Willing to try it. It seems to be coming from the drain though.

Does the floor drain has a built in cleanout plug, is that plug loose? Do you have the washing machine or a utility sink in your garage and are those pipes filled with water at the bottom of the U bend?

Plug is in the basement. The p-trap in the drain is full of water. Nothing in the garage.

I too was about to tell about the hot water thing. It worked for my mom, she didn't try out peppermint thing but the hot water hack helped her. Also, you can try out not using the drain for a day (if possible) so you will know that issue is with the drain or is something else.