On my Gammill I had to use the M brass bobbins, the silver ones didn't work. Tried them and gave them away. I know others that use them on their Gammill but they just didn't work on mine.
I wind my bobbins 'firm' I think squishy might mess with the tension.
You will get use to the noise an almost empty bobbin makes. It is a good indicator of when to change or 'pay attention' to your machine.
The last time I bought brass bobbins they were $3 each but that was years ago. I bought them on line, did my research and got the best price I could find.
ps: I just sold my Gammill yesterday but this was the experience I had for 14 years.