Student or Learner
What is the function of "sailing" in "Let's go sailing" ?
Is it a participial adverb or a gerund ?
Let us go sailing.
Let us go home.
Let us go to the cinema.
Let us go there.
IMO there are instances where 'to' is obligatorily omitted. The covert prep together with its complement (gerund) performs an adverbial function.