Pros
First, I have to say: Gun.io is progressively getting better in a short time, and consistently spending the time to evaluate and retrospect. They have made it a priority to be better than their competitors. They're great to work for if the operational aspects of freelance are too much to bear, as they abstract most of those responsibilities away from freelancers.
Cons
Gun.io is not as wholesome as they seem. They care about developers, yes. They also care a bit too much about their bottom line. We live in a capitalist nation, I get that, and there's no reason why a company shouldn't focus on profitability and growth. However, they often claim to "hate that developers are treated as commodities" but still hold on to many of the same practices as any other headhunter. "Smart and under-utilized devs make us rich" is their mantra, and Gun operates no differently than any other recruiting agency in that regard. The biggest problem I had, however, was the lack of transparency between freelancers and clients. When the project goes south, the freelancers take the hit in reputation. I witnessed many private conversations with clients where the project was hemorrhaging money. In each case, the developers were the scapegoat. For not wanting to treat developers as commodities: they did just that every time. They are not transparent with budgets so developers aren't privy to the burn rates or cash flow issues of clients. Transparency and price gouging is even more problematic when the client is a small startup with a lot of personal investment in the project. They'll often invoice a client at $160-220/hour paying the developer a fixed $100/hour. Oh and don't invoice for meetings, that costs too much money for the client. Instead, developers should swallow that cost instead of Gun taking that out of their margins.