Both players were absent from the squad during Monday's preparations but were present for the opening session at Chelsea's Cobham Training Centre on Tuesday morning.
Gittens' return is particularly significant after the winger spent more than a month sidelined with a hamstring injury. His participation in training places him back in contention for selection for the European fixture.
James, meanwhile, was an unused substitute during Chelsea's tense FA Cup victory over Wrexham. The club captain is now expected to start against the French champions as Chelsea prepare for a crucial Champions League encounter.
Elsewhere in the squad, Cole Palmer, Moises Caicedo, and Enzo Fernandez were given the night off but are all anticipated to return to the group ahead of the trip to Paris.
Chelsea are still without Estevao, who remains sidelined with a thigh injury.
Another notable sight at Cobham was Levi Colwill, who has missed the entire season with a serious knee injury. While he remains unlikely to feature this campaign, his return to training has raised hopes of an earlier-than-expected comeback