You do toontasks which allow you to start training for the gag track. After you complete that toontask, you have to complete 16 other toontasks which can be easy or hard. After that you get the track but have to train to go up gag levels.
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The Brrrgh
There actually is a 16th gag track. I just happened upon it in my own toontown account. However, it says you have to become a member in order to do it. I found that you just have to go to the npc and get the task(its a very long but easy task).
probably the vacum lure gag
When you've finished all required tasks on a playground, including the first 15 gag track tasks, you'll be given a final task (actually, a series of tasks) to do. Finishing that gives you your gag track.
You can't trade gag tracks.