To unlock this one you have to get from bubble gum distributors spread over the map the 5 Gobble Gum from your current pack. The gum from the distributors are random, cost 500$ each and you cant get more than one per round. In addition to the active distributors are moving over the map like mystery box.
Hence as soon as you encounter a distributor working get a gum.
You just need to get the gum from the distributor, you do not have to use it !
Off course, as pointed out by MikeProcurator
, you have to do it during one game !
Trick from https://www.trueachievements.com/FunK+Kaleb.htm
: All the machines will become active during firesales (all gobblegums) and price is reduced to 10 points could be worth adding.Update 1.08, April 2016
: Now all the Gobblegum machines are always working and you can get more than 1 gobblegum per round but the 2nd and 3rd Ggum are very expensive. So stay in the staring area and stick to 1 Ggum per round and you should get it by round 5 or 6.