Try reading this, seems he had the same issue as you.
I have been struggling with Gmod for the longest time not being able to connect to servers like I used to years ago until today where I finally found my long wanted solution. Before explaning the issue I need to address something: Bandwidth and Throughput are two different things in the...
steamcommunity.com
If it didnt work anything from that post try doing this.
1. Go to steam library.
2. Go to gmod properties and click on lauch settings, it should be on the first page.
3. Type this "+clientport 27006" it was stated on a few other posts it fixes an issue like yours.
If none worked some helpers might reply soon and help ya!