See the tutorial:
[Guide] Hosting a Gemini Capsule with your Raspberry Pi
The problems I encountered were related to the hostname: Usually the hostname of the raspi is "raspberrypi". You should use this name for the rsa-key:
openssl ... -subj "/CN=raspberrypi"
I executed the openssl-command directly in the correct dir (/home/pi/bin/.)
Also in /etc/systemd/system/agate.service the hostname should read accordingly:
ExecStart=/home/pi/bin/agate ... --hostname raspberrypi ...
Assume there is a /home/pi/gemini/index.gmi-file you should be able to access it via gemini://raspberrypi in Lagrange or amfora or Elaho or ...