Server Hardware
Hard drive/SSD
In a best case scenario, your machines should be using SSDs, which are significantly faster at reading and writing than HDDs, which use moving parts, making them slower, and more prone to failure.
If you have to use an HDD, make sure it has fast read/write speeds!
CPU
Gmod can only run on a single CPU core, so throw any consideration of buying a CPU with loads of cores and great multiprocessing clock speeds out the bloody window. For your Gmod server, it doesn't matter - it's only able to use one of the CPU cores. So use a CPU with the best single-core performance you can find!
If you're running MySQL on the same machine, it can still utilise the other CPU cores, so still take this into consideration.
RAM/Memory
Because Gmod servers are (currently) 32-bit, they can actually only use a maximum of 4 GiB of memory, so you don't need to go overkill on RAM.
Your best bet is to choose RAM with a high clock speed, e.g. 3200 MHz.
Network
It's probably not a good idea to run your Gmod server on your home network - not only will it be bogged down by your other devices, or the fact that you might even be using WiFi, but home broadband connections are typically just very crap. For example, my house has an Internet speed of 2 MB/s. At university, I can get 25 MB/s in my accommodation (over x10 the speed!) and 125 MB/s in the computer lab.
A data centre has a dedicated broadband connection fit for this purpose, and can reach ridiculous speeds which allow your server to send and receive packets as fast as possible.
Server Location
My players are typically European
Host your server in Europe.
My players are typically North American
Host your server in central North America.
This will provide the best possible latency to those living on both the east and west coasts.
However, if your players are typically exclusively from the east or west coast (this is obviously quite rare), choose a host nearer to those coasts.
My players are typically from the Middle East
I haven't seen any Middle Eastern server hosts in my time, but if you do know of one, you should probably use them.
Otherwise, you may as well host your server in Europe.
My players are typically Russian
If they're from Western Russia, you should host your server in Europe. Internet censorship can be a problem for scripts which use DRMs hosted by CloudFlare or outside Russia.
If they're from Eastern/Central Russia, you should probably look into using an Asian host which doesn't have Internet censorship.
You can totally use a Russian host with no problem, Internet censorship shouldn't affect it too much, but most scripts on Gmodstore which use a DRM will probably not work due to Internet censorship on CloudFlare free plan IP addresses.
My players are typically Asian
Host your server somewhere in Asia, closest to your target demographic.
Also please take into account Internet censorship and firewalls, it's probably not a good idea to host your server in China if you want to attract players from outside China or use scripts with DRMs hosted in Europe and America.
Server Host
And of course, you should choose an appropriate server host which takes all of the above information into account. From my personal experience and what I've heard from customers, I believe that the best Gmod host in the space right now is Physgun.com :D
