don't they both mean hitting it
That'd be á b(h)ualadh. Bailiú is the VN of bailigh, collect.
The difference is best thought of as the difference between the English infinitive (to collect) and the English present progressive (collecting).
Generally, to form the infinitive in Irish, you just use the VN. However, if it has a direct object, you front the object, and a and lenite.
bailiú (to collect)
é a bhailiú (to collect it/him)
The other is the present progressive, which is formed with ag and the verbal noun (though in native speech the g is not pronounced before a consonant).
- ag bailiú (collecting)
However, you can't have a pronoun following a progressive structure, since all nouns following it must be in the genitive. So what you do is you change ag to do (though it remains ag in Munster), add the correct possessive pronoun after it, then mutate the VN accordingly. Note that do a because á (pronounced dhá in some dialects) and that do ár is dár.
- do mo bhailiú (collecting me)
- do do bhailiú (collecting you)
- á bhailiú (collecting him/masculine it)
- á bailiú (collecting her/feminine it)
- dár mbailiú (collecting us)
- do bhur mbailiú (collecting y'all)
- á mbailiú (collecting them)
However, in the example above, they have roughly the same meaning (see ALDB below), but they're formed using different structures.
Yes, "to collect (something)" is an example of the infinitive. In English, the infinitive of a verb is usually expressed as "to (verb)", in Irish, the verbal noun is used (bailiú in this case).
The direct object of an infinitive in Irish comes before the verbal noun, so "to collect it" where "it" is the object, becomes é a bhailiú, with the object é first, then the preposition a and then the verbal noun bhailiú (which is lenited because of the a).
The description on GnaG is correct, but it isn't relevant to this particular sentence.
"chun + verbal noun", (chun péinteáil or chun iascaigh or chun tarlú) implies readiness to start the action described in the verb. But in this sentence, it's tá sé chun é a bhailiú - it's just a straightforward directional/intentional chuig.