If you are experiencing connection issues with ZARP servers (connection failed after 4 retries), and wish to play now rather than waiting for a more permenant fix, all you have to do is:
Go into your Steam library, right click on Garry's Mod and go to properties. In the general tab click on "Set Launch Options..." and type the following into the box:
Then simply press Okay and play!
If at a later date this fix stops working for you (if it worked in the past) remove the clientport command from the launch options and then launch Garry's Mod. If it still doesn't work quit Garry's Mod add the command again and then relaunch Garry's Mod and it should work.
I can not guarantee this will work for all of you but it will work for most of you.