Gargano is a priest of the Red God, and he is devoted to his God. When he speaks about Her, he refers to Her as 'our God' even to those that don't believe in her, because he firmly knows that H'staier is the one and only God.
Gargano adamantly believes in Sheer Haert and that Rhaenerys is Sheer Haert, he thinks that the dragons she and her family have are gifted to her by the Red God. Gargano doesn't do much, admittedly, but it is interesting to see how he views people. Syror Qostoff, an unholy man who thinks God has abandoned him, is challenged by Gargano. Gargano poses that H'staier has not left his side, instead it is he that has left Her side. So his view of people is that they have abandoned the side of thee one true God, but H'staier remains by their side. He manages to convince Syror by showing him a fraction of H'staier's power, by conjuring up a fiery live image of Rhaenerys Blackfyre.
We also see the way Gargano views criminals, he doesn't matter who they are - their age, skin whatever, because he judges these people all the same. He doesn't make difference between a child and an old man, because he doesn't want to be favourable or pick sides - in his mind he and H'staier judge people fairly by using this metric. Syror and him argue about this, with Gargano even failing to understand Syror's point, thinking that there is no point in looking at two criminals differently.
It will be very interesting in the 3rd book to see if Gargano is right about Rhaenerys - if she is Sheer Haert, and what would happen if she isn't? Gargano sees life in black and white, so what would happen if Rhae isn't what he thinks she is?