"Take on"should be "Take on the job" to form the prepositional phrase.

I may well be misunderstanding Gatsby's example, but I would say that "take on the job", meaning "accept to do the job" is actually a phrasal verb followed by its direct object. The only way that I can see "on the job" as a prepositional phrase in this example is if "take" is a noun as well, so that you could ask someone "So, what is your take on the job we have been asked to do?"

Phrasal verbs have to be the brainchildren of the devil, at least for language learners...

Marianna