I share your concern.

Since the response body is not encoded, there's no safe end-of-response marker byte(s) to use.

So content-length seems like the way to go. But knowing content-length ahead of time is difficult for dynamically generated content (CGI is supported after all), so they also need something similar to HTTP chunked encoding, which does complicate things a little.

I understand that keeping the Gemini client simple to implement is one of their design goals, but I don't think the same is true for the Gemini server. So I hope that they would consider adding these to the protocol. They could probably stuff the content-length or the word "chunked" in the <META> string.