County take full control
The visitors were dismissed before tea for only 153 after losing the toss, Jon Lewis taking three for 34 and Steve Kirby, Anthony Ireland and Vikram Banerjee each picking up two wickets.
In reply, Gloucestershire, again without skipper Alex Gidman because of a back problem, had moved to 144 for three by the close, Ireland skipper William Porterfield making 53 and Craig Spearman 36 not out.
Spearman, in the captain's role, saw his decision to field vindicated as Middlesex plunged to 80 for five at lunch.
Nick Compton was caught behind off Lewis for seven and Andrew Strauss, changing his mind over a pull shot off James Franklin, top-edged to Banerjee at mid-wicket.
Eoin Morgan (six) was bowled off an inside edge by Kirby, who followed up by having Owais Shah caught off a top-edged pull by Kadeer Ali at deep square for 25.
When Neil Dexter edged Lewis to Hamish Marshall at third slip, Middlesex were in deep trouble.
Ireland removed Gareth Berg (29) and Chris Silverwood, while left-arm spinner Banerjee weighed in with the wickets of Shaun Udal and David Burton. Lewis bowled David Nash.
Porterfield and Kadeer Ali put things in perspective with an opening stand of 62. It ended when Kadeer fell lbw to Dexter.
Porterfield played some fluent shots in reaching his half-century off 99 balls, with eight fours. He fell soon afterwards to an edge off Finn.
But although Marshall also departed for 18, Spearman ensured it was clearly Gloucestershire's day.
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