Both "darkness" and "gloom" have basically the same meaning and are used to emphasize how dark the prison was. See how you translated this in [Psalms 107:10](./008.md). AT: "complete darkness" (See: [[rc://en/ta/man/translate/figs-doublet]])
Here the word "Oh" is used to convey a strong desire for people to praise Yahweh. The abstract noun "faithfulness" can be translated with an adjective. See how you translated this in [Psalms 107:8](./008.md). AT: "Let people praise Yahweh because he is faithful to his covenant" or "People should praise Yahweh because he is faithful to his covenant" (See: [[rc://en/ta/man/translate/figs-exclamations]] and [[rc://en/ta/man/translate/figs-abstractnouns]])
Both of these phrases describe Yahweh freeing his people from prison and are used to emphasize that Yahweh truly freed them. AT: "He freed his people from prison" (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-explicit]])