Don't add gelatin to hot liquid. Soften in 1/4 cup of the milk, then . . .Heat milk in a double boiler to a temperature just below boiling. Add the gelatin, sugar, and a pinch of salt, and stir until everything is dissolved.
Beat the egg yolks until they're frothy Blend a small amount of milk mixture into egg yolks (so they won't curdle). Gradually add the yolks to the milk mixture in the double boiler, stirring constantly. Continue cooking and stirring until custard begins to thicken.
Set aside and cool to room temperature. When cool, add yogurt and vanilla.
Refrigerate the mixture for a few hours or overnight.
When you're ready to freeze the yogurt, beat the egg whites until they form soft peaks. Gently fold them into the chilled yogurt mixture.
Add to your ice cream maker and freeze according to the manufacturer's directions.