Gray joined Bayer in January from Leicester City before returning to England this summer.
The Blues star remarked: "It [moving to Germany] was difficult at the beginning.
"But I took myself out of my comfort zone and had to mature. My girlfriend came out after a while and I began adapting to the culture and German life.
"I learned a lot from it. I recognised I am a home person and need my family around me. In Birmingham, I had my mum and nan and uncle 15 minutes away, so I could relax.
"Out there, you go to work, get back, eat, sleep and repeat.
"It was toughened up my thought processes and turned me into more of a man, I think. And the way things have worked out is perfect.
"If I chose the other option it wouldn't have been possible to make it happen when Everton wanted me.
"Growing up, if someone mentioned Everton I'd think, 'Yeah, big club, good club, when Goodison is bouncing it is really bouncing'.
"Coming here was an easy decision."