From what little I have read about GBN, I'm not sure you can Id the problem positively based on only symptoms. Often the basal plate will show damage and the bulb will have a shrunken appearance. I think you need to send plants/bulbs to have them tested but I think some Ag colleges in the NE including Cornell offer free or low cost testing. The GBN may be present in your soil and not cause a problem at first but then over the course of a few seasons the population will build up and wipe out your crop. Not sure if it is more prevalent in sandier soils like Root knot nematode.

Then the buggers would stay in the soil forever??

For many years yes. They would decrease naturally over time if no alliums were present for them to feed on. Bio-fumigation can also be used to put a dent in their population. Certain types of mustard greens are reported to be effective in decreasing the infestation.

To get rid of them, all potential host plants would need to be eliminated from the infected area... and there are more hosts than just onions & garlic. The Cornell site lists some of the other known host plants.

Prevention so much better than cure. Rotate, rotate, and use those mustard greens.

No point in worrying about what you have no reason to believe you have.... if you start googling all the things that your plants might get or your soil might have you will drive yourself nuts pretty fast.

Of course, that doesn't mean you shouldn't take proper preventative measures for the future. But, I chose not to buy certified garlic and bought from a local grower instead, as the cost was just not really reasonable.