sounds like the amaranth was definitely overcooked...
If you cook the raw grains in water, it is going to be a sticky texture, overcooking leads to the gruel you are describing, which is actually a fun thing to play with with butter, roasted garlic, grated cheeses etc. to sub for mashed potatoes with your protein. And amaranth also makes a great polenta sub. It's also great stirred into soups for thickening and nutritional boost, and cooked with cream, butter, cinnamon, berries, nuts, etc. as a cooked breakfast cereal like oatmeal or cream of wheat.
Back when I was eating a lot of whole grains, I usually popped amaranth (yes, it pops just like popcorn). You can also combine equal parts of amaranth and arborio rice in a risotto. The creamyness is a bonus here.
The popped amaranth can be used in so many ways: sprinkled on foods for added nutrition, salad sprinkle, garnish, stirred into meatloaf. There are millions of ways to use it this way.
And of course, finally, it is added to baked goods, whole or ground into flour, to add amazing nutritional benefits to bland old wheat flour when baking.
Amaranth. So good and good for you.