I always enjoy playing Gleneagles (Kings or Queens--the Nicklaus course is another matter entirely......
. Nevertheless, on the Kings:
--too many holes have an obvious and relatively easy line for the drive. Not a lot of thought required on the tee.
--greens tend to be lacking in any interesting movement. A fair amount of slope on some, but mostly unidirectional.
--green complexes themselves generally have more bark than bite. The volcano hole looks intimidating, for example, but getting up and down from a mishit tee shot is not usually a great challenge
--The course is short and plays short. However, to toughen it up they have to make it really fast and firm, which makes it even shorter, which is not really acceptable to today's punter, so......they make it soft, and longer, and grow in the rough, which makes it even less interesting, strategically. See the left hand side of the 1st in the pictures above. This hole did have some strategic interest when the proper line was to flirt with the the bunkers there--which are now effectively out of play.
The course can be spectacularly beautiful. There are some very interesting holes on it (2, 6, 11, 14 and 17 come to mind). The hotel is worth visiting by itself, at least for a drink or lunch if you can't afford to splurge and stay there. The Scots love the place and would probably prefer to play there than Muirfield or TOC. But.........
........in the top 20 GBI courses? No way, Jose!