Additional Director General (law and order) Anuj Sharma said the police had information about Gurung's links with Maoists and the rebel groups.
"He has procured arms from them. We have recovered six AK-47 assault rifles, one 9 mm pistol, 500 rounds of ammunition and bombs from the place where a police party was ambushed by his group near Darjeeling today," Sharma told reporters here.
He said the police faced "heavy firepower" from the group which led to the death of a police sub-inspector and injuries to four police personnel.
The police had the information and when they raided the area, Gurung and his group attacked them, the ADG said.
Earlier reports said GJM supporters had clashed with the police party.
Gurung was now trying to get back to Sikkim, he said.
Sharma said a cornered Gurung was trying to foment trouble in the hills again.
"We will not allow this to happen at any cost, even if we have to make sacrifices," he said.