No it isn't a device itself. It is a ship with early teleport prototype. Ship can have nuclear reactor, that's why energy on the ship isn't a problem. But it failed. Why? I don't know, but it seems, ship disappeared in 1970s. That's why I think, Aperture scientists miscalculated something...
It seems, portal gun simply creates wormhole through higher dimensions space. This process takes much less energy than other teleportation method, but you need manually create portals on both wormhole ends. You can't use this method to travel between worlds.
As for Borealis - we know almost...
First of all, English is not my native language, so I hope, that text below is readable. Please help me to correct my “translation mistakes”. And please fell free to share this text on other forums (I didn’t find many Half-life active forums. Maybe you find them? But write to this topic, if you...