Here's how the roll over appears to work for comped or status-matched cards and I suspect that gold from credit cards will work the same, but am not sure:
With a matched Gold card, you still need to stay a full 75 nights to reach Concierge, not just the difference between gold & concierge, i.e., 40 nights. Similarly, for roll over with a gold card, no nights will be rolled over unless you have actually stayed more than the requirement for gold, i.e., 35+ nights. Same principle applies for the other levels. This was my experience for both roll over and reaching higher levels. I also contacted their customer care at the time and they confirmed this. Having said that, you might have a different result! "YMMV".