I carry a pump regardless of what and where I ride. CO2 is a one-shot deal, and besides, CO2 won't last long in a tyre anyway. I also don't see the point of CO2 with larger tyres than narrow road tyres.
Probably obvious, but the point of CO2 on gravel is to seat the bead if you can "fix" the tire without a tube.
But certainly, 90% of the time I'm using a small pump to keep the air flowing until the sealant does its job. For instance - a pinch flat will seal if I get sealant to pool there. Its not going to seal unless I stop the bike and tilt the tire so the sealant goes near the rim. A CO2 would often just blow the sealant out - slow application with a hand pump does the trick for me.
And, when I've broken the bead seal and reset it - CO2 is the only thing that is going to work out in the middle of nowhere.