Gardiner stands at 10-under-par 132 at StoneWater and is one clear of both Keith Nolan (65) and James Driscoll (68).
Kris Cox (65), Jason Day (66), Joe Daley (67), Jeff Klauk (66) and Hunter Haas (67) are knotted in third place at 8-under-par 134.
Gardiner began on the back nine Friday and collected his first birdie at the par-5 13th when his 20-footer found the bottom of the cup. He hit a 6-iron to 15 feet to set up birdie at the 15th.
The Australian missed a 14-footer to save par at the par-3 17th, but atoned for the error one hole later when he tapped in for birdie at the par-5 closing hole.
Gardiner collected his first birdie of the second nine at the par-5 fourth. At the par-3 sixth, Gardiner hit an 8-iron to 3-feet and holed that birdie effort.
His final birdie came at the 390-yard, par-4 eighth. Gardiner hit a lob wedge to 2 feet and kicked in the birdie putt for the halfway lead.
'Just had a great day,' said Gardiner, who has a pair of top-5 finishes in his last two starts. 'I couldn't have a better day. Not much wind out there. Any mistakes were all your own fault.'
Josh Williams and Ryan Hietala each fired rounds of 7-under 64 on Friday. They are tied for ninth place with Brock Mackenzie (67) and David McKenzie (69) at 7-under-par 135.
Roland Thatcher, the first-round leader and winner last week in New York, only managed an even-par 71 and is now part of a group tied for 14th place at minus-6.
The 36-hole cut fell at 2-under-par 140 and the Nationwide Tour's leading money winner Paul Claxton missed the weekend with a two-round score of even-par 142.