Galiano is the most easiest and most obvious to romance. Just keep doing the repeatable tasks to keep his friendship level high (green one) so other conversation that lead to his romance would come over next in his set of text box, and you need to really finish main quest in order to placate his romance, or you'll just just be staying behind. By means, main quest, you need to. Spoiler alert! Finish the:
-Eradication of Sub'Guhnirs (at the end of the mission he will ask you if you'll do anything he says, if you chose ,"sure" (you'll become his bitch, less respect on you), and "not sure" (he'll respect your choice becoming his love interest).
-The mission of Battle of Power "where you will choose Galliano over Garth and the same mission involving of killing Garth for the first time or cause why not? You want Galliano to like you right? (Do not take Garth's offer, Spoiler:Garth will not die for the first time even if you kill him.)
-After Defeating Arihama (he like the idea that a hero (You/Mc) is under his wing).
-Going to the Second Floor of Akra mission is, you'll be his body guard, when he meets a scientist about a product called Etherium, GIVE GALLIANO THE ETHERIUM) Cause Phoenix will offer you to steal the etherium for Galaxy Credits, hence, betraying Galliano's trust.
-Clash of Corporation (this main quest is mostly he was coming in join with cause you're going to meddle in his boss' affairs. If he trust and respect you he'll take your suggestion).
Despite Galliano's mafia-like conduct he is a good guy that listens to you once you gain his respect by NOT being a bitch (pertaining dominant choice of your MC).
Ps: 'Being bitch' means your MC is submissive towards Galiano.
Ps: Galiano and Galliano (double L) is the same person. It is also a lexical representation in the game which both shown in the game. (If you have a screenshot of the names I would love to add it here). I never heard about it as wrong cause both names are in the game itself and not been written to be corrected or recognize as error, so for me both names are correct in my knowledge.
If there are questions please feel free to ask for clarifications thank you.