gerund?

It is all three. You frame the question as if the three possibilities are exclusive. "Fishing", the gerund form of the verb "to fish", is the direct object of the sentence.

Many thanks, Contrex.

Fishing for a meal can be fun.
(Fishing is a gerund in this sentence)

I am fishing
("am fishing" is the verb)

He likes fishing more than skiing.
("fishing" is the object of likes)