"I do not need to learn anything else."
Translation:Ní theastaíonn uaim aon rud eile a fhoghlaim.
There is a certain overlap in the phrases, but to me, there's nonetheless bit of a difference between "ní gá dom..." and "ní theastaionn uaim..." The first, to me, come across more as "I don't have to / am not compelled to" and the second doesn't have that implication of compulsion. I could need to do something, but not have to, in the sense that no one will make me do it if I choose not to... If that makes any sense at all....
A nominal in English just means a noun, or a pronoun, or noun phrase, or anything acting like a noun. Phrases like 'to learn something' (verbal noun phrases, where it's in the name!) are nominals in English. For example, you can say, 'to learn is a godly act' - because it's in the subject position, you can tell it's a nominal. I guess it must not be the same in Irish, if you can't put a VN phrase between teastaigh and ó.