It is very safe as it uses a spare microSD (which should be empty ) to install the vulnerable mset.
But of course you're still screwing with sysNAND, and even tho technically nothing can go wrong here you never know.
man i was so scared my emunand would mess up .. luckily nothing happened.. and bro when i used your method after i put the launcher dat on both the msd the one i have inside my n3ds which i have my cias and emunand in, and then i also put it in the formatted spare msd that i usually use for my gw card.
i did the following right after , i put the formatted msd which only has the devmenu.3ds file and the launcher dat, i put it inside my n3ds internal slot.
then i boot gw menu with my oot and then installed that vrn thingie ,right after that i placed my gw card inside the cartr. without any msd inside of it and then i boot gw mode but nothing happens black screen only :l.
i hope there is a safe way to get this to work, i followed the instructions of the Gateway Site perfectly :L but not doing it for me , meh.