...anytime I was out in the garage flys would come in a buzz around as soon as the doors were opened...It got annoying real fast!...
On the opposite side of the fence of trying to kill the flies is preventing them from coming in when you have the garage door open. There are a few places that make what is basically a large screen that attaches to the garage door opening, helping to keep them out while you work inside. Here is a picture of what I am talking about:
It makes your garage into a big screened in room. The screen goes up and comes down pretty quick, and there is even a version that has a seam running vertically down the middle that you can push open and walk thru. The seam is held together with some magnetic strips so that they close back together automatically after you walk thru.
I don't know if anything like that would work for you, but it's out there. I am thinking of getting two of them for my two car garage, because I also like to work out and have the doors open when the weather is nice. But it seems like as soon as it starts to get dark, and I turn on any lights, holy cow! Flies and bugs everywhere!!
And the worst is when I am working on something that takes requires both hands, and one of those damn knats gets into my ear!
Jim