Gorou build recommendations
Gorou is a Support character that can give increasing buffs to your party members from his Elemental Skill and Elemental Burst based on the number of Geo characters in your team (capped at 3). Casting his Elemental Skill sets up a banner which gives flat DEF (1 Geo), Interruption Resistance (2 Geo) and Geo DMG% Bonus (3 Geo) for your active character standing inside the banner radius. Casting his Elemental Burst creates a field following the active character which deals periodic damage and offers the same bonuses given by the Skill, destroying any placed banners in the process, it also pulls in nearby Elemental Shards created by Crystallize reactions. Due to his kit focusing on buffing DEF and Geo DMG, he's a specialised support geared towards Geo DPS that scales with DEF, such as Noelle or Itto. He can see niche use at [C6] with other Geo characters as well.
Regarding Weapon Choices
Elegy offers a helpful ER substat alongside an EM and ATK% buff for your teammates. While not directly useful in teams centered around Noelle or Itto, this could possibly be a good choice for future Geo DPS characters, or in a team which utilizes Geo Resonance.
Regarding Artifact Sets
Casting Elemental Burst regenerates a total of 6 Energy for each of your party member. This set reduces your Energy Requirements by a bit for all of your party members, which can be quite beneficial. Do note that this is a 4 star set so you're potentially missing out on 5 star stat scaling, which is a 34% difference in mainstat value and 25% difference in individual substat value.
The effectiveness of this set increases if you've already met the ER requirement for Gorou. At that point, you'd no longer need to run the recommended ER% sets, which means you can opt for this instead.
Regarding Artifact Mainstats and Substats Priority
DEF% mainstats and substats (and potentially a Healing Bonus mainstat on Circlet) can be considered at [C4] due to being able to heal the active character based on his DEF during burst. This is useful if you plan to use him as a solo healer.
Crit Rate circlet and substats can be considered when Gorou is using Favonius Warbow to trigger the passive comfortably.
ER% substats can usually be more important than getting the right mainstat, as Gorou's damage contribution to a team is negligible, while being able to get Energy for his Burst is a lot more important.
Artifact stats priority
er def% crit rate