The Gumball Machine is the final boss in the Tower's standard progression.
The Gumball Machine appears in the center of a large, circular room, and briefly after entering the room, it will protect itself with layers of armor plating. It has a wide variety of attacks, mostly consistiong consisting of plasma orbs (or "gumballs") that damage the player on contact. Each color has a special movement; yellow balls orbit the Machine at ground level, purple orbs will home after the player like projectiles from the Betty Turret, and so on.
As the Gumball Machine takes damage, it will gain more plates of armor and spawn more defenses, such as spinning spike wheels on the floor, a ceiling platform mounted with Lasers, and start spawning Napoleon tanks.
Keep moving, try your best to dodge the various defenses and destroy enemies that get in the way, and always watch for an opening in its armor plating to deal damage. Since enemies don't drop pickups, avoiding damage is key.
Before the third set of armor plates covers the Gumball Machine, it is possible to back up to the edge of the room and shoot over the tops of the plates at the very top of the weak spot. An accurate gun, or gun with the "precise" mod helps a lot here.
Once the third set of armor plates covers the Gumball Machine (2 overlapping walls around the outside, and 1 set of plates over the top), you can shoot the colored "globe" above the machine to open all the armor plates towards you, but shooting the globe will trigger an explosion that will hurt you.
If you have enough jump height or extra jump pickups, attacking the Gumball Machine with a high damage gun from above will make the fight MUCH easier and quicker.
Also, if you have the Blue Shell item, you can instantly kill it by trapping the shell in the armor plates.