Heat a large skillet over medium-high heat. Add the frozen gyros and fry for 7-10 minutes, breaking up the meat as it cooks, until heated through.
In a small bowl, stir the onion soup sachet with roughly 1/2 to 3/4 cup hot water to create a thick paste, not soup.
Add the soup mixture to the gyros. Reduce heat to medium and cook for the time indicated on the onion soup packet, stirring occasionally. Add 1 tablespoon of water at a time if the mixture becomes too thick.
Drain the mushrooms. Add the sour cream and mushrooms to the gyros, stirring to combine.
Heat for 2-3 minutes over medium heat until simmering, stirring occasionally. Do not boil, as sour cream may separate.