A gekko is running up my leg.


Senior Member
German/English bilingual
If you want to say that a gecko is running up your leg, then yes, you need to include "is".
The version with "is" is a sentence.
The version without "is" is not a sentence, it is only a clause.
Such a clause could, for example, act as an object within a sentence: "I felt a gecko running up my leg."


Senior Member
English - Southern England
Yes! Unless it is the answer to a question such as "What made you run away screaming like that?":D [The answer to which would be a colloquial way of saying "The fact that a gekko was running up my leg]