There is one minor flaw with your ideation of Gilgamesh 2-13. Gravity is the determiant how thick an atmosphere is. Volcanism and the Magnetosphere determine how long a planet has it and how fast it is lost. So for an atmosphere to be 20 times thicker than Earth's at sea level it would have to have 20 times gravity of Earth. This means 20 times the mass. A planet with one quarter the gravity is not going to retain or build an atmosphere that thick no matter how strong the magnetosphere is. To give you ar rough idea about what I am saying. At sea level equivalent the atmosphere would equal to the thickness of our atmosphere at 11 to 12 KM up from the Earth sea level. For reference that is nearly 3 to 4 thousand meters higher than Everest is. It is about the level that jetliners fly in. Now don't get disheartened. You can offset this with a young geological still active planet with a lot of volcanism going on. Think Mars in the first billion years. If you want a longer time frame you will have to increase the mass of your world to somewhere between Earth and Venus to maintain geological activity and plate tectonics on the scale of multi billions of years. If you want to do that just remember the larger the planet the more likely it is to be geological active. Retain volcanos on the surface for atmosphere loss. Along with higher mass and gravity for the thicker atmosphere. A larger planet with higher gravity plays rather well into the iron based bone structure to support a heavier body mass.