To every gar I got I only offered frozen-thawed aquatic cuisine, cut if needed, and pellets, preferably floating. They are greedy, voracious living fossil pikes and do learn to eat whatever's offered. They do prefer thawed, fleshy foods.
Given the small size of your gar and that it is accustomed to live foods, you may not be able to easily break it, that is essentially fast it long enough for it to cave but once it is ~6"+, I don't think it'd be a problem. Starting from good health, one would have 1-2 months to "convince" the gar to take dead foods but usually it would not take more than 1-2 weeks without any special tricks, such as garlic, rubbing new food in old food, using tongs with old food, then with new food, dangling food on a thread, etc.
There is a section on MFK on gars.
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