The gemini specification requires UTF-8 support, though there are other encodings out there. In testing, some random gemini clients either perform "garbage in, garbage out" or may warn about alternative encodings. A test server might help with this:
https://thrig.me/src/scripts.git
$ od -bc latin1
0000000 102 374 143 150 141 162 012
B 374 c h a r \n
0000007
$ doas ./gemini-encoding-server 1234 \
/etc/ssl/localhost.pem \
/etc/ssl/private/localhost.key \
iso8859-1 latin1
A localhost certificate may help with testing.