A word that takes the base form of a verb, and adds "-ing" can be either a gerund, or a present participle, depending on how it is used. Such a word can be used to function as a noun, in which case it is a gerund. Or it can be used to function as an adjective or as an adverb, in which case it is a present participle. Or it can be used to form a sentence using the progressive tense or progressive aspect (both terms are used for the same constructions). In this case some call it a participle, and some just call it an "-ing form".
A gerund derived from a verb usually describes the act associated with that verb. For example, when one swims, one is performing the act of swimming. When one watches, one is performing the act of watching.
If you are spending your time watching TV...
includes two -ing forms. The second, "watching TV" is indeed a gerund. It describes the act associated with the phrasal verb "watch TV". It functions as a noun. (Because gerunds are derived from verbs, they can have objects, so "TV" is here the direct object of the gerund "watching", and the two together function s a noun, the name of the activity.)
The first "-ing" form is part of a progressive present construction "are spending", a verb form describing an activity now in progress and continuing, or one that is habitual.
So one -ing is used in the example to form a progressive, and the second is used to name an action.