Changing it is generally used when you host multiple servers (as each server has to use its own port when using the same IP).
You can change it to anywhere from 27015 to 27020.
“-port 27015” This is the default server port. Garry’s Mod can only handle up to 4 players. No matter the distance between them, any object which passes through one portal will emerge from the other and vice versa instantaneously. The ASHPD is a hand-held device which has the ability to manufacture two linked portals. How does the portal gun work in Portal 2? The portal gun’s full name, Aperture Science Handheld Portal Device, can be abbreviated as ASHPD, which resembles a shortening of the name Adrian Shephard, the protagonist of Half-Life: Opposing Force. It takes about two seconds for the projectile to travel this distance….Portal Gun. The Portal Gun has a maximum range of 675 blocks (1350 feet). The Aperture Science Handheld Portal Device (also known as the Portal Gun, ASHPoD, Aperture Science Quantum Tunneling Device, or ASHPD) is the experimental tool designed to place two portals that objects can pass through. This is by far the most eloquent and bug- free portal gun available within Garry’s Mod.