Both clubs have already qualified for the last eight, but Bath occupy pole position and victory over Recreation Ground visitors Bucharest would secure the number one berth and a probable home quarter-final against 2001 Heineken Cup finalists Stade Francais.
Any slip-up, though, and Gloucester would overtake Bath by defeating Kingsholm visitors Mont-de-Marsan.
Wasps and Perpignan are also through as pool winners, while Stade Francais need a point against already-eliminated London Welsh in Paris to confirm their progress.
The five Amlin group winners will be joined by three runners-up from Heineken Cup pools for the quarter-finals in early April.
Bath University student Tom Dunn makes his debut against Bucharest, packing down at hooker alongside front-row colleagues Charlie Beech and Anthony Perenise, with lock Stuart Hooper returning to captain Bath.
Gloucester rugby director Nigel Davies has named a strong side that includes England internationals Freddie Burns and Charlie Sharples, with Scotland lock Jim Hamilton as skipper.
"If you look at the groups and how they've panned out, there is no doubt in my mind that we've got the toughest group with two Top 14 teams and another Aviva Premiership side in London Irish," Davies said.
"So, for us to be five (wins) from five at this stage, we can be very pleased with that.
"But it's important that we finish on a high note with a big performance and a win which allows us a home quarter-final, which would be a fantastic occasion."
- Sports & Recreation