I think a GPS tracker is a good idea, but a visible deterrent, such as a heavy chain should be used as well. While battery grinders can cut through cables and locks relatively quickly, the idea is to have the thieves pick an easier target.
A chain or cable should be long enough to loop through an immovable object, otherwise the thieves can just pick up the bike and throw it in the back of a truck. A friend lost a brand new motorcycle this way. He had a lock that immobilized the bike, but did not tie it to anything. A security cam video showed a van parking next to the bike. Four guys got out, picked up the bike, threw it inside, and drove away. The whole thing took a few seconds.
