The top seed's hopes of being one of the eight men to play in the lucrative tournament at the O2 Arena on the bank of London's River Thames appeared to be ebbing away as he fell a break down midway through the final set in Moscow.
However, the 27-year-old clung on in an eighth game that stretched to seven deuces before breaking back to level for 4-4.
That appeared to take the fight out of Kukushkin and Gasquet wrapped up his 10th title when his weary opponent slammed a forehand wide.
The win allowed world number 10 Gasquet to leapfrog fellow Frenchman Jo-Wilfried Tsonga in the race to secure a place in the Tour Finals.
"It was an incredible week here for me and now I have great memories about my playing here," Gasquet told reporters.
"It is one of the most successful seasons in my career but the race for a place in London is not over yet and I need to concentrate on my further playing to get there."
Rafa Nadal, Novak Djokovic, David Ferrer and Juan Martin Del Potro have already qualified for the tournament while Wimbledon champion Andy Murray has pulled out following back surgery.
Tomas Berdych, Stanislas Wawrinka, Roger Federer, Gasquet, Tsonga and Milos Raonic are in a battle to secure the remaining four spots, along with Tommy Haas.
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