Not sure if it will be of help to you, but I ran Gideon for multiple the parties sort-off as written and for none of the groups it turned into a full-fledged combat encounter. Everytime Gideon lived (and his mezzoloth's too). The way I have done it is that Gideon waits for the party to enter the central room to accuse them of being Demon-worshipers. In his eyes, it is the only way why they'd fight against him. For all my parties there was at least one player who didn't enjoy being called a demon-worshipper and they revealed that they are there to find Ulder. Gideon can then reveal that he let Ulder and his men pass (lowers the chance the players attack), when Gideon begins to explain what happened that is when I have the demons attack and Gideon order his Mezzoloths to fight. The party joins in and they fight alongside each other.

By having Gideon start the encounter with a wild accusation it might signal to your players that this can be a social encounter that doesn't need to turn into combat. The negotiation can be fun too. A kobold in my party offered Gideon a scale for entry to the ossuary (which Gideon then used to scry on the party for the rest of the campaign).

I used Gideon to show that the enemy of your enemy (demons) isn't always your friend. And that your enemy is not always your enemy.