A derivation of the 'blowjob' a slag term for the act of fellatio. The primary difference being that the objective of a 'blowjob' is to blow one's load, whereas the objective of a 'gagjob' is to use one's erection to gag the provider of oral stimulation.
The gagjob rose in popularity through 1990s but was not formally recognized until after Sept 11, 2001. When through frustrations over our inability to capture the elusive Osama Bin Laden, we turned to choking women with our cocks.
The gagjob has risen to the level of 'preferred filth' for many porn enthusiasts leading to the creation of websites such as; gagsgirls.com, makethemgag.com, the gagging blog, and deepthroat links to name a few.
That girlscout got such an intense gagjob that she puked.
The skank's eyeliner was running after the gagjob; either from tears, sweat, or shame.