I use the single-side stages crank arm power meter on my bikes for a few reasons.. they work pretty reliably and they're a more affordable alternative when opting to just use the non-drive-side single arm. Customer service over at Stages has also been great for myself and many others
I got a single-leg Pioneer PM for my road bike last year, on deep discount from Competitive Cyclist. I'm sure you'll get a million different recommendations... just wanted to share that this option has worked well for me!
I currenlty use a Quarq D-Zero on a 2016 Trek Domane 4.5 Disc. No complaints, works fine, service and support are pretty amazing. While mine isn't a gravel bike, I'm pretty sure this thing is rugged enough to survive all but a rather harsh enduro or downhill crash. The system is fairly well sealed, and the battery is easily accessed and replaced. Calibration and zeroing are also extremely easy. If you've got a phone with Bluetooth, download the Qalvin app and it'll walk you through calibration. If you need to zero at any time, simply spin the pedals backwards 5 times and the meter will zero itself.
Another vote for Stages here... I've been using them exclusively for the past 4-5 years and their customer service has been exceptional. Just be sure to buy a V2 or V3 crank arm. The V1 had some problems with water getting inside and the battery doors falling off.
I currently have an older 2016 model dual sided pioneer crank powermeter on my time trial bike. I really like it, becuase it is super data heavy. I have knee problems and i can make sure my balance is equal and i'm not pushing to hard on the better leg. Only issue is that it wont connect with my garmin headunit. So i had to buy the pioneer headunit. The newer pioneer systems do sync with garmin headunits so thats great.
My friends us the garmin vector pedals and they have hit or miss connectivity with their garmin headunit, but the pedal based systems are great and easy to move bike to bike.
I will probably buy a stages single sided like the other posters for my next purchase, since that is really the only data that I need.