This Gary's MOD - Prop Hunt is not standalone, meaning that it can't be launched separately. According to the developer's information you have to install the mod from the Store through the Subscribe button and then after the game is launched, you will be able to select the gamemode.
There is also a FAQ page on the Steam page where you can get additional answers to questions to avoid any other issues that might appear.
Reminder: Downloading is possible only after the Subscribe button is pressed.
