In an opinion piece in Monday's edition of The Washington Post, Gass, NATO's senior civilian representative in Afghanistan, said: "The Taliban have taken a ferocious beating over the past year" in their former strongholds in the southern and southwestern regions.
"They are demoralized and finding it harder to resupply with men, money and weapons. The fighting campaign they boasted of this summer has been a flop."
Gass credited the success to the U.S. troop surge, and other NATO and international partners providing the means to defeat the insurgency.
Citing an example in October, Gass said U.S. Marines in Helmand expected a tough battle as they prepared to take their operation to the north to the strategic Kajaki Dam where there is a main supply route.
"I feared heavy casualties when we set out. But that didn't materialize. The Marines met relatively slight resistance as they cleared and secured the road to the north. Most of the Taliban fled rather than fight," Gass wrote.
He, however, warned the successes do not mean the coalition forces have won as plenty of work remains to be completed.
"But make no mistake: Taliban commanders in Quetta and Peshawar have plenty to worry about. Nobody should think they are winning or can have any expectation of winning. The Afghan people don't want them back."
Gass also noted progress within the Afghan army, adding NATO will need to continue its highly effective training program.
"If we tighten the purse strings too early, we will risk all the gains we have made at such cost. That would mean a return to the security chaos that bred attacks on our countries."