Gloucestershire 119 for 4 (Leach 3-29) trail Somerset 591 for 7 dec (Abell 142, Renshaw 94, Gregory 89, Lammonby 76, Hildreth 53 by 427 runs
The visiting skipper took his run tally in his last four knocks to 415 with a measured 142, made from 246 balls, with 17 fours and a six, as his side took their first-innings total from an overnight 319 for 4 to 591 for 7 declared.
Any hope the hosts had of escaping Somerset’s grip disappeared at the start of the day when Abell, still on his overnight 52, edged Zak Chappell straight to second slip where Miles Hammond somehow contrived to spill the chance.
Gloucestershire’s already lengthy injury list was extended when skipper Graeme van Buuren crashed into an advertising board attempting to prevent a boundary and had to leave the field nursing a badly jarred right shoulder.
Ryan Higgins assumed the captaincy as Abell and Steve Davies batted with caution through the first hour, the latter bringing up 350 with a back-foot forcing shot for four off Brad Wheal.
The total had progressed to 361 when Davies was caught behind driving for 37 to give loan recruit Wheal his first Gloucestershire wicket.
Gregory’s arrival saw Somerset accelerate immediately as he lifted left-arm spinner Zafar Gohar for a straight six.
A swashbuckling shot through the leg side for four saw Gregory bring up 400 before another boundary off George Scott took the all-rounder past 3,000 first-class runs.
Abell, who must be on England’s radar, played his part in raising the scoring rate and reached a 202-ball hundred with a single, courtesy of a misfield.
Gregory smashed the next delivery from part-time spinner Jacob Bethel straight for six and by lunch the pair had added 92 off 90 balls.
The afternoon session saw Gregory go to fifty with his third six, a pull off Chappell, and it was just a case of how many the visitors wanted to score.
Abell was eight short of his career-best score when top-edging a sweep off Zafar to Wheal at deep square, while Gregory robbed himself of a hundred, run out by Wheal calling for a third after Craig Overton’s bottom edge past the wicketkeeper.
Some lusty blows had taken Overton to 35 before Abell called a halt in mid-afternoon. There were 11 overs before tea and the first ball of the opening one saw Peter Siddle pin George Scott plum lbw.
Having kept wicket for 147 overs, James Bracey found himself walking out to bat at three with hardly a break. He and Harris took the score to 33 by the interval.
Bracey rode his luck at times, but also hit 6 fours in reaching 39 before pushing forward to the final ball of Leach’s first over and edging the England left-arm spinner to Overton at slip.
Leach struck again as Marcus Harris, on 32, guided the last delivery of his fifth over into the hands of Tom Banton at short-leg and grabbed his third wicket when Hammond edged a back-foot defensive shot to Overton.
It was an impressive effort from Leach, who bowled at just the right pace to apply pressure on an unresponsive surface.