In a slow cooker, season the frozen hamburger patties with garlic powder, salt, and pepper. Then, put the patties in the slow cooker (overlapping so everything fits in one layer).
In a separate bowl, mix soup, one can of water, and gravy mix. Then, mix well. Pour the mixture on top of the hamburger patties.
Cook for 5-7 hours on low or 2-4 hours on high.
optional: As long as the hamburger steaks haven't been moved while they're cooking, grease should build up in the middle of the slow cooker (between the patties). A ladle can be used to slowly skim the grease from the center, then throw it away.
Remove the hamburger steaks, then cover them with a towel to keep them from getting too hot,
Ladle out about 1/2 cup of broth/gravy then cover slow cooker immediately and increase temperature to High.
When you add cornstarch to the 1/2 cup of broth/gravy that you kept, mix it well until it's a smooth paste. We don't want the sauce to be too hot.
Pour the cornstarch slurry into the slow cooker, whisk until it's smooth, then cover and cook on High for about 5 to 10 minutes, or until the sauce is thick.
Add hamburger steaks back into the gravy and then serve it to your guests.