First off, you would need to set up a DVAR which you can call from your menu to the gamemode you are trying to add.
For example:
if(!(getDvar("GameTypes")== "Avalanche"))
{
setDvar("GameTypes", "MainModMenu");
}
if(getDvar("GameTypes") == "MainModMenu")
{
setDvar("GameTypes", "MainModMenu")
}
if(getDvar("GameTypes") == "Avalanche"))
{
setDvar("GameTypes", "Avalancuhe")
}
This would go under init()
You would also need to make a function and call it to change the gamemode. You would do this by calling the dvar to change the gamemode.
Keep in mind, you cannot have functions that are the same but have different uses, for example, you cannot call avalanches init() and yours at the same time.
And the way you would call / revert back to your main menu would to be just to call the dvar using a function.
I hoped this helped!