Your first question in the forum title (Kasparov's skill largely came from computers- true/false?) and the last question (was he just average without computers?) is decidedly silly/false, since he became champion in 1985, well before grandmaster strength chess playing computers were available.
Kasparov did become champion while the Soviet Union was still around and the Soviets had decided advantages before computers because the Soviet Union had long supported chess financially and had teams of strong grandmasters thinking about chess openings and trying to find innovations that could be useful and building manual databases of opening research. (Some of the fruits of which can perhaps be seen/found in the Informant series.) Whether Kasparov got full access to such research after he became champion is questionable because the Soviet system began breaking down soon after he became champion.
Questions about the history of the use of computers by grandmasters to find/check opening innovations might be very interesting. I found one article about Nakamura's use of computers in his preparation that I found interesting.
Similarly, the history of their use in endgame analysis would be interesting to read about. The elimination of adjournments came soon after the time that computers became strong enough to assist in endgame analysis.
Here is a link to a review by Kasparov of a fairly recent book on AI and chess he doesn't seem to like very much. Kasparov's review covers some of the questions you ask.
I've read online that Kasparov was the first player to use computer chess databases and that much of his dominance came from this.
Several questions:
1.) What GK really the first to do so (at least among the top players)?
2.) What did such usage consist of? Were these databases like engines in providing moves to consider and analysis of positions?
3.) Why didn't others do the same around his time or did they?
4.) Lastly, and most importantly, was GK just an average top player without such computer assistance? E.g., Could others have beat him (like Fischer and the old-timers) if he didn't have access to computers?
5.) ...Thanks for your answers!