All high protein foods are low GI. GI measures effects on blood sugar and only applies to carbohydrates. Protein has little to no effect on blood sugar so foods high in protein with no carb content have no value at all on the GI scale. In fact adding protein to a meal is a way to lower the overall GI content.
Whey powder is possibly the best and most easily assimilated source of protein available. You can buy it at health food shops and either have it on its own mixed with water, or add it to milk-shakes or smoothies.
If protein isn't keeping you full and you want to incorporate more low GI carbs into your diet you can check this list of the GI values of foods
. Just pick the lowest ones on the list and they should keep you full for longer. Always bear in mind calorie and fat contents, however. Adding fat to foods also lowers the GI so potato chips are technically lower in GI than baked potatoes, but they're not much of a food for losing weight!