Panchal struck 122 off 256 balls and found the fence 17 times during his knock after Gujarat opted to bat at KSCA Rajnagar Stadium.
Skipper Parthiv Patel failed to impress, getting dismissed for 12.
Panchal's opening partner Samit Gohel made 87 off 207 deliveries, and the two added 172 runs to lay the foundation for a big total.
Gohel struck 11 boundaries and two sixes, before getting out 13 runs short of a century.
At the close of play on the first day, Manpreet Juneja (22) was giving Panchal company.
Mumbai's star seam bowlers Dhawal Kulkarni and Shardul Thakur went wicketless.
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