Run true multiple process instances of gnome-terminal

Due to the new client/server architecture (details here) you'll have to start separate instances of gnome-terminal-server. To do that you could use the --app-id switch:

/usr/lib/gnome-terminal/gnome-terminal-server --app-id my.first.Terminal

then use the client (with the same app-id) to create terminals¹ inside the new server :

gnome-terminal --app-id my.first.Terminal
gnome-terminal --full-screen --app-id my.first.Terminal
......

Another instance:

/usr/lib/gnome-terminal/gnome-terminal-server --app-id my.second.Terminal

and terminals:

gnome-terminal --geometry 80x24+200+200 --app-id my.second.Terminal
gnome-terminal --app-id my.second.Terminal
.....

Path to gnome-terminal-server might be different on your system.

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^{1: Note that "you have 10 seconds to use the client to create a terminal inside the new server".}

According to man gnome-terminal, the option you're looking for appears to be the confusingly-named

   --disable-factory
             Do not register with the activation name server, do 
             not re-use an active terminal.

However, the option is apparently removed in more recent releases so should not be relied on.