I heard 건지 is used for emphasis, but I'm not entirely sure how to use it and I don't know what it means when I use it.
I hardly say 건지 is used for emphasis. It is used to express wonder or uncertainty.
As for 거야, is that like です in Japanese (it's kind of just there to end the sentence)? In my book I see that 거야 indicates the future and is really informal. Is this true? Could 거야 be used for present tense or only future?
In most cases, 거야 is used in the future tense because it indicates a speaker or a listener’s plan or intention.
Please see my above examples.
However, it can also express a speaker’s guess and in this case, the tense is present, of course.
아마 늦은 봄이었을 거야. I guess it was late spring.
Is 거나 "or"? In my book it says it indicates "or" and can be past, present, or future tense.
Yes, you are right.
Would I be able to use 거나 with friends that are slightly older than me or should I use them with friends that are younger than me?
Because 거나 is merely a connective particle, you can use it with anyone regardless of their age.