A little tricky. You are right in saying that "checking" here is acting as a noun. Simplifying the sentence gives us "Checking renders them less efficient at work." (As a side note, it should be "renders" with an "s" for the singular, since the act of checking is a singular noun.) However, it is not just "checking" but *frequently* checking, and not just checking, but checking *cellphones* - so "frequently checking cellphones" becomes an entire "phrasal noun." (This is not the actual definition of a phrasal noun, but let's just use it for now to describe this phenomenon of a group of words acting together as one noun.) In other words, *within* the phrase "frequently checking cellphones," "checking" acts a verb and therefore is modified by an adverb. However, the entire phrase "frequently checking cellphones" acts here in this sentence as a noun.

A gerund has both noun and verb characteristics. "frequently checking cellphones" = gerund phrase, subject of the sentence. frequently = adverb for the gerund "checking" checking = gerund of "to check" cellphones = gerund object, direct object of the gerund "checking"

@Jocob. very good example. learned

It’s correct. Look at this. Quickly cooking the food Quick cooking of the food

Checking in this sentence is a verb so there is not need to describe it wth an adjective.