It would be a verb, just like "running" is a verb.
Examples:
He was running forward.
He was lurching forward.
There was a mechanical problem in the car, it kept was lurching whenever you turned it on.
Retreat
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.