This one has bothered me so much that I actually phoned a friend of mine in Scotland who is a bit of a planespotter to ask what actually happened.......His recollection (which shows how actual history and recollections often diverge!). A few tidbits:

1) The exclusivity of Prestwick was in fact repealed in 1990, after having been delayed several times.
2) Several airlines did in fact announce at the time that they would start service into GLA in 1990.
3) There was a delay of several weeks, even months, as the current GLA T2, was NOT completed in time.
4) There were some "inaugural" flights conducted in 1990 HOWEVER the Gulf War changed things significantly, with several airlines cancelling such plans - and in fact most flights to Europe, and some of these services did not in fact really kick off properly until late 1991 or even early 1992.