Are you using the IP: mc-gtm.net
Are you using Minecraft Java Edition and are you a premium account (or cracked/pirated)?
I have a legit copy of Minecraft Java Edition, and I'm using 1.15.2. I did use mc-gtm.net, but it never did pickup the server
Try going on 1.12.2 and using the IP.
it still happens no matter what.
It shows a red x and server network is unreachable