Could a gravel bike be the ideal Audax Bike?

['Kaze Pete]

With a max average speed of 30km/h of an Audax ride, the rolling resistance might not be such a big deal.

Spending 8+ hours in the saddle (25km/h avg) might prioritize comfort. I'd say any tyre from 38mm and larger (including 50mm, or even Dylan Johnston inspired 2.2s).
Heck, I did a 150km tar ride the other day on my hardtail MTB (front shock - only shock - locked out).
The MTB was wearing 2.35 in the front and 2.3 at the back. Never felt like my tyres were holding me back. The tyres were pumped for the occasion though. 

[Pieta Swanepoel]

Just make sure that the design of the frame fits your body type. Some gravel bikes are more racier than others. Or the top tube is short/long/etc.

I for one don't like the fit of a Giant. But it works for millions of other people all over the world. 

What I am saying is to testride the bike. Tires are relatively cheap to replace.

[Bonzaai Rider]

Agree to the topic of comfort over speed. Been riding Audax last few years on my mountain bike with clip on TT bars. Knowing that speed is given up changes the outlook on the ride. 

But no matter what bike you choose , just being out there doing endurance is to be admired. 

Enjoy and good luck. 

[monsterINC]

Completed my Super Roudeneur this year all done on my gravel bike with 40mm, found it to be a perfect setup

[Dexter-morgan]

Build yourself a lekker Monster Cross bike, MTB with drop bars and you won't look back.