OP, I'm going against the grain here, I think you've had an unnecessarily hard time on this thread... Many people are saying that they'd be upset/annoyed/outraged at being told they had to stay at the wedding venue, however, these people are not your close friends and family. If anyone close to me rang up and said "we're getting married at Gleneagles, we'd love you to be there. Everyone needs to stay there. We know it's expensive, so we can pay half of your room cost. Do you feel you will be able to come? We understand if you can't. We insist on no presents" I would move hell and high water to try and be there, and in no way would I feel anything but delighted to be invited.
Have I got it right that the wedding will be October 2017? That is nearly 18 months away. Surely, your close friends and family will get to know the venue long before the invites go out? They will have plenty of time to decide what to do. If it were me, and money was an issue, I would save in between now and the wedding. Just £10 per month would be enough to cover the cost of the room. (I'm sure some people will say "well I couldn't spare £10 a month" but only you will have an idea whether your close friends and family could).
I also think anyone who wouldn't attend simply out of principle of having to stay there are just cutting their nose off to spite their face. They would be getting to stay in a luxury hotel, at a discounted price, spend time with friends and family and see two loved ones get married. To pass up on that just because of the venue's rule seems, well, just a bit petty silly to me.
I know there will be individual situations that might be tricky, but on the whole, do you think you could talk to your intended guests and explain to them? Others might be aghast at that suggestion, but these are your close family and friends! Not some strangers on the Internet!
I also think some people might think you ABU if they don't know much about Gleneagles, thinking, 'Oh, it's just a posh hotel'. Well, it is a posh hotel, but it's also much more than that. We have stayed there twice, I can totally understand why you have your heart set on it.
I think you sound really lovely, OP. You aren't blindly dictating things to your guests, you are taking their needs into consideration. I don't think you are being unreasonable or a bridezilla. Talk to your close family and friends and see what they say. Good luck! can I come to the wedding? I will happily pay for myself