There are (I think) 3 levels of garage gym flooring.
LEVEL 1: The first level is getting yourself 4 x 6 horse stall mats.They are affordable and effective! These cost about $45 each. After I placed my order at rogue for my squat rack, I went to the tractor supply store and purchased four of these mats. They are 3/4 inches thick, so they are extremely durable and protect both your bumper plates and garage concrete floor. Relative to garage concrete floor, the mats do damp the sound a bit. So that's a plus. And yes, you can park on top of them!
LEVEL 2: The second level is to add carpet tape underneath your mats. I bought carpet tape from home depot for under $10. Personally, I still park in my garage so every time I would park the mats would move ever so slightly. I found myself moving the mats prior to every workout. Eventually, it just got annoying. So if you want them to stay in place get some carpet tape. Overtime they will move a bit but it's so much better than before.
Note: There is one benefit to not taping and that is you could reconfigure the mats if needed. For instance, sometimes I would pull one of the mats out. This would happen when I am using the squat rack and I needed additional space for a second bar or a different movement.
LEVEL 3: The third level is to tape the mats together. When I first heard of this, I wasn't a fan of taping. I didn't like seeing the lines and I thought the taped areas would be slippery. It wasn't until I visited a friends garage gym and saw it in person. It looked good and it wasn't slippery at all. I bought gorilla tape from home depot for about $6. Prior to taping, during certain workouts the mats would move away from each other but not anymore.
Extra Credit: If you park on the mats get an absorbing mat. I think I got mine Autozone for a few bucks. This will keep your mats clear of any oil drippings. I always move it out of the way before a workout and once I'm done, back it goes.