is it when you try joining a server?
if so its steams validation check to see if you actually own the game, in this case im guessing you got it off this site, so in other words you dont actually own the game. anyways heres a short list of some of the common servers that are cracked and that you can join with both the name of the server and ip address enjoy
:
Garry's Mod Non-Steam Server
garrysmod.ru Build server C - 79.164.43.6:27017
garrysmod.ru DarkRP server - 79.164.43.6:27019
garrysmod.ru Stranded Server B - 81.9.3.134:27016
garrysmod.ru ZombieSurvival server - 79.164.43.6:27016
.:`=-~rANdOm~`-=:. -Parasite|Tick100|Fast DL- - 173.60.111.163:27016
.:`=-~rANdOm~`-=:. -Winter Survival|Tick100|Fast DL- - 173.60.111.163:27017
.:`=-~rANdOm~`-=:. -Zombie Survival|Tick100|Fast DL- - 173.60.111.163:27025
.:`=-~rANdOm~`-=:. -Build|Tick100|Fast DL- Plain! - 173.60.111.163:27021
.:`=-~rANdOm~`-=:. -Build|Tick100|Fast DL- Latest Wire/Phx - 173.60.111.163:27015
(For more .:`=-~rANdOm~`-=:. servers click here)
Garry's Mod RP - 69.9.43.105:27015
GMod.LT || Build - 79.98.29.134:27020
Super Mario Boxes - 76.108.93.32:27015
King John's Server - 68.117.139.33:27015
Nebtown Sandbawxin - 24.68.133.128:27000
GrafischX.net | GMOD Server - 85.92.138.197:27015
[United|Hosts] DM |LotsaWeapons|PHX|FastDL| - 92.0.199.87:27015
●● IRIZ Network ●● Experimental *BUILD* Server ●● - 88.191.73.231:13372
GDS New Server [WireSvn|PhxSvn|HammysPropPack] - 75.108.143.140:27015
^Eco$ervPoland^ |OPEN 24/7|NON-Steam|Tick100|Serious DarkRP| - 83.223.105.76:27015