Gillis, 48, has made more than $5.2 million in 188 career PGA Tour starts with two notable results. He tied for second at the 2012 Honda Classic alongside Tiger Woods and behind Rory McIlroy, and he lost in a playoff to Jordan Spieth at the 2015 John Deere Classic.
But Gillis never won at the highest level, and he wasn't able to keep his card during the 2014-15 season despite his strong play at TPC Deere Run. He spent last season bouncing between the PGA Tour and Web.com Tour, but this season only has conditional status on the developmental circuit.
Gillis has two younger children at the school, and he told the Detroit News that he views the coaching opportunity as a possible bridge until he becomes eligible for the PGA Tour Champions in July 2018.
"I thought it'd be good for me. It keeps me playing, and it keeps me competitive," Gillis said. "As I told them, 'I'm not done.' I'm not 100 percent sure I'll do it for more than one year. We'll see how it goes. I've never done anything like this before."
Gillis has not played in an OWGR-rated event since a 73rd-place finish in September at the WinCo Foods Portland Open on the Web.com Tour, although last month he finished second at the Jamaica Open.