asataḥ mā sadgamaya
tamasaḥ mā jyotirgamaya
mṛtyoḥ mā amṛtamgamaya(Bṛhadāraṇyaka Upaniṣad 1.3.28)
In sadgamaya, jyotirgamaya, amṛtamgamaya as @haritirumalai pointed out, sad, jyotir and amṛtam are all in the accusative case (2nd), and appear in the stem form; and as per sandhi rules in compound with gamaya - which I mentioned earlier was a causative imperative, 'cause me to go, lead me.' As you rightly pointed out, gam takes the accusative, so that's taken care of. Because of the stem form, the case ending that was confusing you, is dropped. (santam santau sataḥ) And all three are neuters. Tamas and jyotis are anyway, and amṛta can be m/n/f, but in the sense of immortality it is a neuter.
Now the first part of each sentence could have be interpreted two ways depending on whether you take mā to be not/don't, or whether you take it to be the enclitic for mām - meaning mujh ko, mujhe, 'me' as I first reacted. But, since asataḥ , tamasaḥ and mṛtyoḥ are all in the ablative case (5th), there is no scope for ambiguity, and @haritirumalai got it right the first time.