I’ve been doing a good amount of gravel and mtb XCM racing this year, and have noticed something interesting. When riding a long dirt road climb with a steady grade, it’s more difficult to achieve the same steady power output on my mtb than a gravel bike. I’m about 10% lower.
The only major variable I can think of is that I’m generally 1000-3000ft higher on mtb rides than indoor and gravel. Would that explain it?
Are you riding a full-suspension bike?
You might be getting some power loss from your rear shock. 
If they’re different power meters, that could be it - they can’t be trusted to read the same without testing them.
Altitude difference could contribute.
You could be more acclimated to the position on the gravel bike
Full suspension bike I think that the power losses are after the power meter - basically there are losses but bars, seat, crank are all “solid” and directly connected.
Could be altitude. The effect of being 3000’ higher is bigger between 6000’ and 9000’ than it is between 1000’ and 4000’.
Hmm…
What color is the gravel bike?
Suspension loses, losses to different riding position, different power meter.
Thanks for the feedback. The riding position differences and altitude make sense.