Their headquarters are local to me. Though I've never visited. State Bicycles seems to be their main competitor to the direct-to-consumer market. Though State has a more comprehensive line-up of steel, alloy, carbon, titanium and both mechanical & electronic groupset offerings. Poseidon probably has an arrangement with Microshift since that's the only groupset brand they offer. I hope they stay relevant, they do hit a consumer friendly price-point with decent components.
I'd say amongst casual gravel rides I've been on; Poseidon, State, All-City and Surly tend to be the most common. Then you have a bunch of old school rigid MTB frames and either the titanium and/or custom steel guys. Carbon is still a bit bougie for the US gravel scene, unless you're a roadie adjacent gravel'ist! 
I've been there a handful of times, picking up bikes, parts, etc. I don't find the people at Poseidon to be the most inviting or helpful bunch, lol. No matter, the bikes are good quality and hit a great price point. I've had two X's over the years and wish I still had the last one, my cousin currently has a tricked out X and a Redwood. They have their bikes on display, I'm sure you can try them out.
Ditto on all the brands you mention. Poseidon, State, Surly, etc. are all over the place. I have a steel Marino frame that I really like and highly recommend. Can't beat their price for custom geo and specs.
I would say carbon gravel bikes are pretty common in the areas I ride, almost equal to the steel/alloy/etc riders.
I was also recently in Mexico for a gravel race and carbon is definitely the most common frame material. Almost all the local gravel riders were on nice carbon bikes (saw a couple chiners as well!). Most of the metal frames were American guys on Crust, All-City and some titanium.
