Shillingford's third five-wicket haul in tests helped to skittle Zimbabwe for 175, with fellow offspinner Marlon Samuels supporting well with 3-15. Fast bowler Shannon Gabriel began the collapse with 2-10.
Zimbabwe captain Brendan Taylor top-scored with 33 while opener Vusi Sibanda (32) and debutant Sean Williams (31) also got starts.
Gayle's solid innings then lifted West Indies to 114-2 in reply. The powerful left-hander forged an unbroken third-wicket stand of 79 with fellow Jamaican Samuels, who ended a good all-round day on 26 not out.
Fast bowler Kyle Jarvis took 2-35 for the Zimbabweans.
Zimbabwe started the day brightly after being sent in by Darren Sammy once persistent drizzle cleared and play finally got going a half-hour late.
Sibanda and Tino Mawoyo added 42 for the first wicket before the strapping Gabriel struck twice in his first two overs to derail the tourists.
Mawoyo battled through 30 deliveries for 8 before he misjudged an inswinger from Gabriel and had his off stump rattled as he played no stroke.
One run later, Gabriel claimed the dangerous Sibanda, the right-hander gifting his wicket by driving a full toss hard and low to Kemar Roach at mid-on. Sibanda struck six boundaries.
Taylor and Hamilton Masakadza rebuilt briefly in a third-wicket stand of 21 before Shillingford, loudly acclaimed by a home crowd of a few thousand, made a breakthrough with his sixth delivery.
Masakadza compiled 14 off 35 balls before he played all around a doosra and was bowled.
Taylor countered the threat of Shillingford with some authentic sweep strokes but perished to a reverse sweep off the same bowler soon after lunch at 105-4. Taylor hit five fours off 62 balls before he was bowled when the delivery deflected off his body.
Left-hander Craig Ervine and Williams provided some defiance in a fifth-wicket stand of 31 before Samuels broke through by claiming Ervine lbw to a straighter ball for 18.
Just before tea, Zimbabwe slumped further as Malcolm Waller (9) holed out to deep midwicket off Shillingford and Samuels removed Graeme Cremer for a duck, Kieran Powell snaring a brilliant, low catch at short leg.
Zimbabwe's last six wickets tumbled for 34 runs and the collapse was completed soon after tea.
Samuels ended an encouraging debut innings from Williams, who top-edged a sweep into his helmet and straight to short leg.
Shillingford, who earned the man of the match award for his nine wickets in the first test win in Barbados, completed his five-wicket haul by removing Prosper Utseya and last man Tendai Chatara, both lbw.
Gayle and opening partner Powell gave the hosts a rousing start to their reply before Jarvis struck twice in two overs to lift Zimbabwe's hopes.
Powell, after striking five fours in 24 off 22 balls, deflected a yorker back onto his stumps, while Darren Bravo departed for his first duck in test cricket, chasing a wide delivery and edging to the wicketkeeper.
But Gayle grew in confidence as the day wore on and he and Samuels pulled the Caribbean side from the uncertainty of 35-2 to within 61 runs of the visitors by the close.
Gayle lashed nine fours and two sixes off 84 balls while Samuels hit two fours and a six off 52 deliveries.
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