No. You don't get it. All Metroids start out as larvae, then they progress through the stages. None are born a Gamma Metroids, Alpha Metroids, Beta Metroids, Zeta Metroids, Omega Metroids, or any other (except maybe Fission).
Yes, the Omega Metroid was born a Gamma. You can see his discarded shell, which is two times bigger than larvae one. Directly from Metroidwiki : "Samus will start to find discarded membranes, likely signs of the Omega Metroid evolving, since the Omega Metroid started of as a 2nd stage Metroid." And you can clearly see that the Omega Metroid is much more powerful than the others, able to reduce Samus' life to critical level. Remember that those Metroids are not naturally born. They were developped by the Galactic Federation.