The main difference is, that passmgr uses authenticated symmetric encryption (aes-gcm) to store the contents in a single file, while pass as you already mentioned stores each secret in a separate file, using asymmetric encryption provided by gpg.
Storing secrets in separate files could leak metadata, especially when using a public git repository for synchronization between devices.
Other than that, you dont have to deal with gpg key management in passmgr.
I did something similar with a project for Node.js called nermal[1], which comes with an example called `tagaloop` that I still use to store my passwords. Even though it had a bit larger of a scope (it is a library for encrypting/decrypting arbitrary JSON with AES-GCM + scrypt + random-length-padding), one of the examples is a password manager called `tagaloop`. It did not really get the sort of traction that I hope you're going to see -- good luck! Fortunately I can still call it a success because it has made my life much easier in many cases.
We both can make different choices and that's OK, but I'm happy to see as a design point that both of our password managers are interactive loops to avoid the history management of the shell. I will say, though, that my files routinely hold way more than 4 passwords and listing them all for the user to choose by number is not really appealing: so one of the biggest things that `tagaloop` does is that it just lets you grep through them at the interactive console. You might want to consider that as a very important feature.
I think there are many users or who dont want to setup gpg on multiple machines. I can also imagine scenarios where it is simply not possible to setup gpg on every machine. Maybe you could workaround such scenarios with portable versions of gpg or something like yubikey.
However i like the idea of having a single binary with a minmal set of dependencies, which can be moved around easily wherever i want to use it.
Storing secrets in separate files could leak metadata, especially when using a public git repository for synchronization between devices.
Other than that, you dont have to deal with gpg key management in passmgr.