Lenovo took the top spot during a quarter in which PC shipments dropped overall due to a weak economy and pressure from mobile devices.
Of the top four PC vendors, only Lenovo was able to grow its shipments. Its PC sales increased by almost 10 percent to 13.77 million units, giving it 15.7 percent of the market, Gartner said.
HP's shipments dropped 16.4 percent to 13.55 million units, putting it in second place with 15.5 percent of the market. Dell was in third place after its shipments declined 13.7 percent, giving it 10.5 percent of the market.
A similar study released Wednesday by IDC had HP clinging to its top spot, with 13.95 million PCs sold compared to 13.82 million for Lenovo.
Lenovo benefitted from building up its distribution channel and from "missteps" by its rivals, IDC said. HP is still trying to get its PC business back on track after reversing a plan to sell or spin off the division. Dell has been moving away from lower-margin PCs and focusing on products that yield bigger profits.
Overall worldwide PC shipments totaled 87.5 million units in the third quarter, down 8.3 percent compared to the same quarter last year, Gartner said.
The third quarter, which includes the back-to-school season and is usually strong for PC shipments, was hit with weak economies globally and by buyers moving to smartphones and tablets for their computing. In addition, a new category of thin and light laptops called ultrabooks failed to spark a revival in PC shipments during the third quarter, IDC said.
Looking ahead, the research firm expects a continued weak market for PCs until the release of Microsoft's Windows 8, said David Daoud, a research director at IDC, in an interview.
The U.S. presidential election on Nov. 6 could also help the PC market, he said, by bringing some clarity to the U.S.'s economic plans and thus encouraging vendors to invest in new product development.
Vendors have been trying to clear out their Windows 7 PC inventory, and demand dropped as buyers waited for Windows 8 to arrive, Daoud said. The new OS could help PC sales start to recover in the fourth quarter and into 2013.
Beyond Windows 8, a recovery in PC sales will depend on the success of thinner and lighter PCs, including products with tablet-like features such as touchscreens, instant-on and longer battery life.
"Revival of the PC market will have to be linked to the way people look at the next generation of PCs," Daoud said, adding that ultrabook prices will need to come down.
Today, ultrabook prices start at about US$600 and reach more than $1,000 for some models. Intel, the primary backer of ultrabooks, has said prices will come down in future years.