There aren't many manufacturers for tadpole tandem trikes. Greenspeed from Australia was one but the owner died and the company name has been taken over by Terratrike. It is not a GS tandem because the frame is much different. You can see a GS GTO tandem on CL
https://inlandempire.craigslist.org/...469973597.html It is pretty easy to spot a GS tandem as the mesh seats were nearly always bright yellow. This one seems to be priced reasonably but has been there nearly a month now. I own a single GS GTO trike and it is right up there with the best quality trikes.
Here is a picture of an older Trident Chameleon trike
https://bicycleman.com/used-trident-chameleon-tandem/ that was sold recently by Bicycleman in Alfred NY for $2K. If like this one, the one you are looking at has the SRAM dual-drive in the rear, it predates 2015 when SRAM discontinued the hub. What makes me think your trike might be an older Trident is the frame coupler that is visible in the last picture. Trident uses at least two of them and perhaps four on their trikes. It's the silver thing in front of the derailleur in the picture.