One moment you're enjoying a nice walk. The next a helium tank is shoved in your mouth and you inflate like a balloon. Least you'll have a good view of the city lights.

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I've had many people ask how I've made these. So lets get technical and explain in short. Its a mod called "PAC3" and it lets you modify your player model in ANY way imaginable. An important note about PAC3. Its only works if you're PM is being "seen", its intended for multiplayer. So for it to work well in single player you need a Third Person view mod.
To create my inflated PM. What I've done is set my animation to none or ragdoll I think it says. Then create some bone modifiers to get the pose right. Set the chest bone to jiggle, so you wobble like a balloon while moving! Next you can add models to your PM, the default sphere will do great. Add shapes to arms and legs to get the fully inflated body! Then you can add a "Player entity" this will allow you to change the gravity on yourself and size too. That's should get you started on doing something like this in Gmod PAC3