How to make a gmod server run 24/7?

You find a host and then you buy a server and it runs 24/7

To keep a server online for 24/7 you'd need to never shut it down and keep your server payments up to date.

what i have is a gaming PC that is on 24/7 so i can run the server on it maybe that is a idea?

Try a hosting service or vpn do lower costs and to run a server 24/7.

Buy a hosting service and put your server onto it. Aslong as you restart your server time from time aka once every day, maybe at a time when people aren't on it or so to refrain from it starting to lag, you should be good.

Simply buy a server from a game hosting company.
I really recommend HexaneNetworks.com to buy from, they're cheap, and can currently get 50% OFF with the code HALLOWEEN2018.

This company do really good servers aswell as deals! I used them for a LONNGG time! Check them out.

ELHostingServices

Dedicated hosting companys such as Crident (my personal favorite) and its not too hard to set it it's just a little bit of a learning curve but overall its fairly easy

If you are like me, I have a VPS which means like a Virtual Machine but its not hosted by me and its online 24/7, i can access it from Remote Desktop connection and it works great, I can turn off my pc and everything and the server will stay online.

You need a game host, I use Hexane because it is by far the best and you can keep your server up 24/7! and there is currently a 50% sale.