Earlier quoted context omitted.
This is a self-signed cert with no chain of trust to a cert issuer, which means we're back to square one... No way to know that whoever is providing the data to Gemini isn't lying about being Conman Laboratories.
Again, that's part of the Gemini spec. > 4.2 Server certificate validation > Clients can validate TLS connections however they like (including not at all) but the strongly RECOMMENDED approach is to implement a lightweight "TOFU" certificate-pinning system which treats self-signed certificates as first- class citizens. This greatly reduces TLS overhead on the network (only one cert needs to be sent, not a whole chain…
OpenSSH's TOFU works in its specific context primarily because SSH targets have been connected to beforehand and you can't place an evil proxy between. In order to prevent issues with TOFU, we've got SSHFP DNS records (so, trusting DNSSEC).
Now with the web, it's already rather ridiculous to suggest that it's not a terrible security model to not visit any new websites on untrusted connections. Plus the seeming lack of alternative methods (like SSHFP/TLSA + DNSSEC) to establish trust with untrustworthy networks (which well, most networks should be considered as).