Hi. Let's say I'm in Yuggoth and I use the Gate Box to return to a R'lyeh gate. Do I get the explored marker for the R'lyeh gate, even though I've only explored Yuggoth? Thanks!
Explored markers for not (fully) explored gates?
If you return to Arkham, you get an Explored Marker. Your scenario is what the Gate Box is intended for.
I'm think a good way of interpreting this is, you get an explored marker for whatever gate you come out of no mater how you come out of it.
Veet said:
I'm think a good way of interpreting this is, you get an explored marker for whatever gate you come out of no mater how you come out of it.
That's not true though. See the official clarifications thread, there is the example of an OA encounter that says "Move to Black Cave". Kevin Wilson made it clear that the investigator does not get an explored marker, even if there is a gate at the Black Cave. The key expression that guarantees you an explored marker is "Return to Arkham".
Good point, but technicaly you didn't return through a gate via that encounter. 
There's something on the other world side of the gate that indicates how to seal that gate. If you can physically shut the gate (Fight) or intelligently enough decipher the indicator (Lore) and have enough experience (Clue tokens) you can seal that portal off from this world. Of course there are other rare or unique ways to accomplish the same thing.
It's not the actual exploration of the Other World that's important for closing or sealing, per se, but the ability to find your way out of that other world (recognize an opening) and seize the moment. In game terms, finding the opening is pretty much automatic but avoiding the delays and dangers is not.
If exploring the other worlds were really key, there'd be more opportunities to earn Clues than there are.