Sorry, you are correct.
This isn't a question with a perfect answer. Some cap-voltage senders will work fine on a GP pin. Some will not. It has to do with the way they output voltage. Only way to find out is to hook it up and see if it works. If it works, it will range from 0-5V. If it doesn't, it will probably be stuck at 5V all the time.
If you have two other cap senders in the plane, you may not be able to do it, assuming they don't work on a GP pin. If you have resistive senders in the plane, you can hook one of those to the GP pin and you cap sender to the normal fuel level inputs.