The 32-year-old former Leeds and Wests Tigers back-row forward was the club's major close-season signing after ending his four-year spell in the RNL and was appointed by Gentle as his new captain for 2013. However, he fractured a bone in his foot during the warm-up to the first match of Super League at Leeds back in February and only resumed full training 10 days ago.
Gentle told a press conference in Hull that Ellis would definitely start Friday's game, along with the club's other major close-season recruit, stand-off Daniel Holdsworth, who has been out for seven weeks with a head injury.
"It's a massive lift," Gentle said. "They are our two key off-season signings. We've been without Gareth from day one and Daniel has only played a cameo role earlier in the year.
"Two players have got to make way and it's given me probably the toughest decisions I've had to make in the 18 months I've been here."
The return of Ellis and Holdsworth leaves only three members of Gentle's senior squad, Danny Tickle, Ben Galea and Joe Arundel, on the sidelines.
Holdsworth, who was signed from Salford, started five of Hull's first seven games but was given an extended rest after complaining of persistent headaches.
"He's now headache free and has been one of the last blokes off the field in every session for the last week so he's good to go," Gentle said.
"It's been a slow process but we've been very cautious, to Daniel's frustration as well as ours.
"We didn't want him back for a week and then gone again. He's back now for the rest of the season and it's the same with Gareth."
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