I think any more than 20% on gravel and you'll be spinning the rear tire if you've not got a wider knobby tire, and coming back down could be a total nightmare.

At 25-27% grade range, I have to keep my chin practically on my bar on my MTB to keep from popping a wheeling and tumbling down the mountain. Anything over 20% and I can't stand and mash or the rear tire will spin. This is with 2" wide somewhat knobby tires @ 34psi. Above 28% or so I start walking the bike.

On pavement steeper stuff is more ridable than on gravel.