Uh, Gidge?
...That is, if she stops tinkering with that machine of hers...
"What?? Huh?... Oh, sorry..."
*clink clank thunk*
"...So, you want to know about 'Gates,' eh? As you may have guessed, there aren't any time machines that'll take you to any point in history with the flip of a switch. It doesn't work that way; that'd be too simple!
"Different eras are connected by weak points in the time continuum... they look like pretty little points of light. When opened, these are called "Timegates." You can only use them if you have a means of opening them, though. Ha, I can handle that! What?... Honest, it won't break... Trust me....
"So, that leads me to another possibility. Paradox! Heh heh. What's a paradox? Well, say you go back in time, and accidentally cause your parents to never meet -- well, ouch! Wouldn't that make you disappear? Yes... but then you wouldn't have been around to change the past, anyway. Paradox!!
The answer lies in how time travel worked when you originally made the jump. There are two types. The first is called a "reality shift," in which you're effected by whatever happens in your own time period, even though you're in another. The second I like to call "matter displacement," in which the subject is completely removed from their own era, and isn't affected by it at all until they get back! And, um, if you ask me, I kinda prefer the second type..."
Well anyway, that's the gist of it. Keep all this in mind when you're reading about eras or thinking of creating a character from a new one. Also, if you've ever played Chrono Trigger, you'll have a better idea of how time travel works here.