"minimum GPA for sports participation"
You wouldn't believe the kerfluffle that caused in Texas[1]. As I recall, the result was rather predictable grade inflation.
Yes, some teachers are hired primarily to be coaches. Some, not all of coaches teach gym class. However, my best teacher ever was a guy called Coach K because he and his father coached football for years. Coach K taught AP physics and engineering courses in high school. He was an Army guy and previously worked at McDonald-Douglass. He made learning fun and exciting. Perhaps Coach K is the exception. Student athletes are there to learn first and a teaching coaches' primary job is to teach.
For example, here is one quote from 2011, something like 25 years after the no-pass-no-play law was passed[1]:
"Dr. Victoria Martin, a Child Psychiatrist in Richardson, TX,...continued by saying: 'Instead of beating these children down even more, we should be encouraging them to participate in activities where they have talents and abilities. We are punishing the “good” kids, the ones who care about their school and want to participate in school activities instead of being involved in gangs and other destructive groups.'" [Emphasis mine.]
As far as teaching coaches go, I'll just mention one of my high-school history teachers, whose primary qualifications as a history teacher seem to have been that he was a very decent football coach.
[1] http://www.adhdtexas.com/no-pass-no-play-law-has-very-advers...