Ross joined Welsh from Saracens in 2009 and has so far amassed 525 points for the club, including scoring two tries.
Ross is one of nine changes made by head coach Lyn Jones to the side beaten 39-17 by Stade Francais in Valenciennes last weekend in the Amlin Challenge Cup.
Tom Voyce and Sonny Parker also come into the backs, while in the forwards Paulica Ion starts at tight head and Tom Bristow at loosehead. Elsewhere in the pack, Kirill Kulemin, Ed Jackson and Adam Balding return to the side, while Michael Hills, a first half replacement against Stade, continues in the number seven jersey.
On the bench, there's a welcome return from injury for Martin Purdy and Daniel Browne, while there's also a place for Neath scrum half Luc Jones - son of London Welsh head coach Lyn - who joins the club on loan.
"One hundred appearances for any club is a huge achievement and honour, and Gordon fully deserves this accolade with London Welsh," said head coach Jones.
"He's been a model professional and looked after himself and his game, and in turn he's looked after London Welsh for the last few years.
"For us to be competitive moving into the final third of the season having a fit and full squad is important and we welcome back Martin Purdy and Daniel Browne.
"We've known for while that we're going to have players ineligible, as well as unavailable through injury, for the LV= Cup so Neath have been good enough to loan Luc to us.
"Playing in Europe was hugely positive, especially for the boys for whom it was their first experience of playing against such high profile sides as Stade Francais and Grenoble.
"We now have the pleasure of playing the European Cup favourites on Saturday. Players need to be able to learn and improve and we we're delighted to be coming down to the Stoop for a third time this season.
"We always get a warm welcome from the home support, who appreciate good rugby, and we hope we can be competitive and to start off where we left off at the Kassam against Harlequins a few weeks back."
London Welsh: 15. Tom Voyce*, 14. Nick Scott, 13. Sonny Parker*, 12. Seb Jewell, 11. Phil MacKenzie*, 10. Gordon Ross*, 9. Alex Davies, 1. Tom Bristow, 2. Dan George, 3. Paulica Ion*, 4. Adam Brown, 5. Kirill Kulemin*, 6. Ed Jackson (c), 7. Michael Hills, 8. Adam Balding.
Replacements: 16. Gareth Evans, 17. Billy Moss, 18. James Tideswell, 19. Martin Purdy, 20. Daniel Browne, 21. Luc Jones, 22. Joe Ajuwa, 23. Tom Arscott.
* Denotes full international
