When you choose an option, a message will pop up in the top-left corner whenever your affection level increases. Please check for that. There are also a few choices in Chapters 1 and 3 that can raise Gahel’s affection.
In Chapter 1, you can increase Gahel’s affection by letting him carry the items for you, and by properly preparing as he advises.
In Chapter 3, you can increase it by taking Gahel’s side.