This doesn't appear to be 100% opensource. The browser and extension are in there GitHub but I can't find the code for their server. I assume the messages are stored centrally?

I have yet to try it out, but the concept is sound. Making every website a "public square", as Gab puts it, flies directly in the face of censorship. However, now we are at the mercy of Gab itself. Gab indeed claims to be free speech, but I've heard rumours that some words will not be processed if you type them out in Dissenter. Nevertheless, we will see in the future how well they hold to that regard.