The Canonet-28 needs a mercury battery to function properly. Without it the shutter speed is fixed at 1/30th and all you can do is set the aperture. Mercury cells are unobtainium nowadays.
With the cell it measures the light and on 'auto' it will choose a suitable (to it) speed and aperture.
If you put an alkaline or silver cell in there with some sort of ring to keep it central in the battery holder make sure it is not in back to front. The wrong way round will show no deflection of the needle, although no harm is done. Alkaline and silver cells (LR44/SR44) will under expose the film as their voltage is 1.55V and the original mercury was 1.35V. You can try a zinc-air cell like a hearing aid battery which is in between at 1.45V. They last a month or so and the results are better. There are adapters which cost more than the camera. They incorporate a Schottky diode to drop the voltage that tiny bit.
To save drain on the cell always switch the camera off 'auto' between shoots as this disconnects the volts from the cad sulfide light sensitive resistor.
I have recalibrated a Canonet-28 for a silver cell but it's not for the un-adventurous. I don't know your skills with tiny resistors and a soldering iron.
When working they are great little cameras. Good luck.