The 25-year-old made his debut back in December 2012 in a PRO12 victory over Edinburgh before signing his first professional contract the following summer.
He made his Scotland debut in November 2013 and has since gone on to surpass 50 test matches for his country.
The former club captain was instrumental in Glasgow’s 2014/15 PRO12 title-winning campaign and will be hoping to help his side take one step towards their first final since then, by marking his century with victory tomorrow night.
Tommy Seymour returns to the starting XV having recovered from a rib injury picked up playing for Scotland. He joins fellow centurions Stuart Hogg and Niko Matawalu in the back-three.
Kyle Steyn shifts from wing to centre to play alongside Sam Johnson in midfield. Ali Price and Adam Hastings once again start as half-backs.
Rob Harley partners Gray in the engine room. Adam Ashe and Chris Fusaro are promoted to the starting line up to play with Matt Fagerson in the back-row.
It is an unchanged front three for the third consecutive game. Oli Kebble and Zander Fagerson once again packing down either side of Fraser Brown.
USA international Tevita Tameilau is included amongst the replacements for the first time since scoring against Cardiff Blues in February and representing his country four times in the Americas Rugby Championship.
Tim Swinson sustained an ankle injury against Saracens last Saturday, although not as serious as first anticipated the second-row is still expected to miss the remainder on the season.
Glasgow Warriors Head Coach Dave Rennie said: “It’ll be a great occasion for Jonny and his family. To have played 100 games for your club and 50 for your country by the age of 25 is unheard of and a testament to Jonny and his work ethic.
“Historically these games have been tasty and it’s great that we’re going to have a packed, noisy stadium at our backs to carry us through. Everyone knows what’s at stake.
“It’s the business end of the season now and we’ve got ourselves in a pretty decent spot but the real fight starts now and the boys are certainly up for it.”
Glasgow Warriors team to play Ulster in the Guinness PRO14, Scotstoun Stadium, Friday 5 April, kick-off 7.35pm, live on Premier Sports 1. You can follow the game on Twitter @GlasgowWarriors. Glasgow Warriors appearances in brackets:
1. Oli Kebble (29)
2. Fraser Brown (85)
3. Zander Fagerson (76)
4. Rob Harley (207)
5. Jonny Gray (99)
6. Adam Ashe (69)
7. Chris Fusaro (C) (162)
8. Matt Fagerson (43)
9. Ali Price (72)
10. Adam Hastings (27)
11. Niko Matawalu (112)
12. Sam Johnson (47)
13. Kyle Steyn (5)
14. Tommy Seymour (127)
15. Stuart Hogg (116)
16. Grant Stewart (20)
17. Jamie Bhatti (42)
18. Siua Halanukonuka (29)
19. Scott Cummings (54)
20. Tevita Tameilau (4)
21. George Horne (37)
22. Pete Horne (154)
23. Robbie Nairn (7)
Unavailable players: Callum Gibbins (head), Nick Grigg (ankle), Huw Jones (knee), Lee Jones (knee), D’arcy Rae (ankle), Matt Smith (shoulder), Tim Swinson (ankle), George Turner (head), DTH van der Merwe (shoulder) and Ryan Wilson (knee).