How do you port forward on a Netgear DG834GSP v3?

I want to forward port 27015 so I can host a Garry's Mod or Half-Life 2: Deathmatch Server. I have looked all over the Internet and I can only get results for a Netgear DG834G v3, which is nothing like mine. Please help me!
Update: I found services but it only gives me the option to put in a port and when a use a tool to check if the port is open or not it says it's closed
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