You clearly haven't hung out with my grandmother.
Schlep can definitely be used as a noun, and commonly is by native Yiddish speakers. (As in "Oy! What a shlep!" or "Picking you up during rush hour would be a shlep, how about you just take the bus?")
There's a natural tendency in English to use as nouns words that were originally verbs. Examples: I'm going for a walk. I'm having a talk with someone.
Dictionary definition or not, a google search for the phrase "a schlep" returns >60,000 results.
If the (recently added) part-of-speech tagging on Google Books is accurate, the chronology is the other way around: it used to be equally used as a noun and verb, but sometime in the 1990s the verb form became dominant: http://books.google.com/ngrams/graph?content=schlep_VERB%2Cs...
A "gerund" is a noun that has "ing" added to it and is used as a verb.
The other way around. From Wikipedia:
As applied to English, it refers to the use of a verb (in its -ing form) as a noun (for example, the verb "learning" in the sentence "Learning is an easy process for some").
Maybe it was just artistic license.