Competition for places in the Fremantle team is extremely tough, with more than 30 players participating fully in pre-season.
Sumich said the 25-year-old Gumbleton, who was traded to Fremantle at the end of last season after seven injury-plagued years at Essendon, had been impressive throughout the summer and was making life tough for the match committee.
"I don't think he's missed a session," Sumich said.
"He might have come off half a session just because we were looking after him, because he hasn't missed one.
"He's been terrific. He'll be an interesting inclusion for us."
Sumich said the round-one side had tended to select itself in previous seasons, but the depth this year was the best he had seen in his time at the club.
"Talking about it with Ross (Lyon) earlier, our first year (2012) we only had eight or nine on the track pre-Christmas and then about 18 to 20 after," Sumich said.
"This year we've had in the 30s (training) for most of the pre-season so it's always going to be competitive for spots."
Scott Gumbleton during a running session, behind another former Don, Kepler Bradley. Picture: AFL Media
Another new recruit pushing for selection is Colin Sylvia.
The former Demon has been kept away from the main group for much of the pre-season as he has managed various niggles, but he joined full training this week.
"He had a slight hiccup with a leg injury but he's fine," Sumich said.
"He's out there and training really well and looking good."
Sumich said Sylvia might not be ready for the first NAB Challenge match against West Coast on February 18, but he could play the following week against the Western Bulldogs.
One man who definitely won't play in the NAB Challenge is Zac Clarke as he recovers from his knee injury suffered on New Year's Eve.
Sumich hoped Clarke would return to full training next week and be ready to play by round one.
"He's tracking along well.
"Hopefully he'll be ready for round one. That's our aim and we'll go from there."
The man who has impressed Sumich the most throughout the pre-season has been captain Matthew Pavlich.
After consecutive pre-seasons plagued by injury Sumich says the 32-year-old is in tremendous shape ahead of his 15th season in the AFL.
"He's done everything, his body is sound; he's looking good."
