The 37-year-old American with an unconventional swing fired a seven-under-par 65 on the Palm course, one of two layouts hosting the final US PGA Tour event of the season, to end the opening round a stroke off the lead.
Gainey birdied two of his last three holes to finish level with Colombian Camilo Villegas, one shot behind South Korean Charlie Wi who also started out on the Palm layout.
Briton Russell Knox, in his rookie season on the US PGA Tour, was a further stroke back on 66, along with Americans Brian Harman and Scott Stallings.
Gainey, who last month won the McGladrey Classic in sensational fashion after closing with a storming 10-under-par 60, was delighted with his start on a blustery day at Lake Buena Vista.
"It's playing kind of tough out there," he told reporters after recording six birdies, an eagle at the par-five 14th and a lone bogey in the opening round.
"The wind's blowing 10 to 15 (mph) steady, gusting up to 20 at different times, and when you catch that gust on the wrong side, you can make some bogeys pretty fast.
"I've got off to a great start, so I gotta focus on maybe trying to win another tournament this week. It's all about winning again for me, because my goal has not been achieved yet."
Gainey's goal is clear-cut. He would dearly love to book a place in next year's Masters at Augusta National by ending the season in the top 30 in the US PGA Tour's money list.
However, victory this week would not be quite enough as the player nicknamed "Two Gloves" could finish no higher than 32nd with total earnings of $2,370,703.
"When I'm playing at Augusta, that means I'm also playing in the other majors, so that's my goal," said the American with the unusual baseball-style swing who occupied 56th spot in the money list coming into this week.
"But I cannot get in the top 30 this week. Not with a win. So my goal is just to win anyway. I'm just trying to play my game and just go from there."
While Gainey is bidding for a second consecutive win on the US PGA Tour, Wi is seeking his first after having to settle for a runner-up spot five times since he began playing on the US circuit full-time in 2005.
"The Palm is a course where you need to score and all the par-fives are reachable, so I felt like I took advantage of them really well," the 40-year-old Korean said.
"What really got me going was number seven when I made that eagle putt from about 15 feet and I felt like I had something special going today.
"I know the scores are low out there, and I'm very pleased with eight under today. I'm hoping this will be a special week."
The top 125 in the money list secure full status for 2013 so this week is a decisive one for Americans Kevin Chappell (123rd), Billy Mayfair (125th), Gary Christian (127th) and Bill Lunde (129th), as well as Australian Rod Pampling (124th).
Among those, Chappell made the best start with a 67 while Pampling opened with a 70 and Christian with a 71.