Finn hasn't featured for Warriors since suffering a bad head injury in last season's PRO12 semi-final defeat to Connacht in May.
He turned out for Ayr against Hawick last week and has gone straight into Glasgow's starting XV for the game against Ulster.
Gregor told BBC Scotland: "He has really relished being involved with the contact side of it in the last couple of weeks,
"We're delighted and it's a testament to the work of our medics and also Finn's dedication and confidence coming back into training.
"He got a real test last week down at Hawick - he had to make some tackles, he carried the ball and his match fitness was tested and he handled it really well.
"You want to know you have gone through the right processes to get you back in a brilliant position.
"We've made sure Finn's had all the time to make that full recovery but you've also got to drive it yourself.
"Finn was always a player who loved the contact side of the game, a very good tackling 10, he loves to carry ball too so he has not shied away from that.
"He's full of confidence. It's a huge test to play against Ulster but we believe he is more than ready for it.
"To have Finn back now means we can attack this game with a player who we believe will be playing most games for us at 10 this season."
Finn Russell's return is all the more timely as Peter Horne is facing a six-week lay-off after injuring his hand in last week's defeat in Cardiff.
"Peter showed his toughness and bravery last week," added Gregor. "I think he thought he'd just dislocated his finger and so he carried on.
"He was probably due a rest, it's just disappointing that the rest is going to take a number of weeks. He's a great leader, a great team man. He is looking at our next opponents, Dragons, so he is going to be heavily involved.
"If there is any player who can come back before the stated date, it's Peter. He will want that hand fixed."