Dear, a server is not necessary when you are trying to create a gamemode, as if you start a multiplayer game yourself, that counts as creating a server, thus you can test your gamemode "at home" so to say. In order to start creating a gamemode, please follow below link.
https://wiki.garrysmod.com/page/Category:Gamemode_Tutorials
Please use same page for references of functions. You may also want to check the base gamemode under garrysmod/gamemodes/base in order to learn how things are done. Please do not modify that, as it is your standard sandbox gamemode files.

Originally posted by Papi:

Dear, a server is not necessary when you are trying to create a gamemode, as if you start a multiplayer game yourself, that counts as creating a server, thus you can test your gamemode "at home" so to say. In order to start creating a gamemode, please follow below link.
https://wiki.garrysmod.com/page/Category:Gamemode_Tutorials
Please use same page for references of functions. You may also want to check the base gamemode under garrysmod/gamemodes/base in order to learn how things are done. Please do not modify that, as it is your standard sandbox gamemode files.

thx m8 that is really help full