"See you later, dear!"
Translation:Ĝis la revido, kara!
"Ĝis la revido" is literally "Until the seeing-again" or "Until we see each other again". (And "Ĝis revido", which I've also heard, would be "Until a seeing-again", with a very similar meaning.)
"Ĝis poste" is literally "Until later".
So "ĝis poste" is probably more for when you will see someone again in a fairly short time - in my opinion, probably even on the same day. "ĝis revido" is more open and could be used even if the seeing-again that you look forward to is further away.
I remember gxis revido from french 'au revoir' which is literally 'at the/to the seeing-again' so 'until next time'. It is the most common fair well in French. Is this the same for esperanton? In French there are also à demain, until tomorrow and à bientôt, see you soon, that are common.
There are esperanto letters under every lesson (small gray bubbles in windows 10), but you can also use this site to find a keyboard: http://en.esperanto.org.nz/how-to-learn-esperanto/how-to-type-esperanto-characters
Why can't I use x instead of circumflex? I can't install an esperanto keyboard
For the same reason that we use "dear" (adjective) in English, and not a noun such as "dearness". It's short for "dear person, person who is dear (to me)" -- "kara (persono)" or similar in Esperanto.