no a gluegun isnt safe for aquarium use. As for goop marine I havent heard of it before so I wont post reguarding that.
is it for a leak or you glueing something together? Be aware your LFS or online pet supplier usually carries Aquarium safe sealant/glue specifically FOR the use in aquariums.
I've used the All-Glass Aquariums Silicone Sealant for DIY CO2 and it performed like a champ. Directions for use are on the packaging and I am sure if you dig around and use your Google-fu wisely, you can find some instructions with illustrations on the internet.