I said Cilic's potential was somewhat limited, I think you are right he could feature the top 8/10 for many years, but he doesnt have the potential to be top 5 or win 2-3 slams, he never had that potential.
Gulbis when I saw him as a 19-year old in RG 2008 and a match against Nadal in 1st round of Wimbledon that year, I thought he would win slams. He had it all, power from both sides, good movement, great drop shots and decent volleys and amazing serve. Well, I think Gulbis might be the worst overestimation I have done in my life but I didnt know his father was a bilionaire, I would have predicted his potential to be somewhat less taking in that factor...
Well, my expectations of Cilic at the moment are many years as a top 10 players with the possibility of winning a Slam. I used to be more optimistic about him when he came into the scene but now I expect no more than this. That said, he should be top 5 at some point. I mean, who is stopping him? Del Potro and Cilic are the only two active players under 25 years of age who even played a Grand Slam semifinal.
Fedal won't exactly last forever, and neither will Djokovic, although he should last longer. Cilic is the second best under 25 active player in terms of achievements and there are not that many youngsters showing superior potential/likelihood to succeed either, so I most definitely wouldn't bet against him breaking the top 5 at some point.
Gulbis is indeed an amazing talent, he's a guy you'd expect to win a Slam at some point. But his Wimbledon campaign this year sums him up really; played a fabulous match to eliminate Berdych on center court and then mugged out in the next round. I'm afraid he will never reach his full potential. Your assessment was correct though, he was a guy one would expect to be very successful on the basis of his talent and game alone.