It's not OP compared to other 1st level spells.
Consider Burning Hands, which deals 3d6 in an area, or Inflict Wounds, which deals 3d10 damage, or Dissonant Whispers, which deals 3d6 while wasting the target's reaction in order to provoke attacks of opportunity, or Chromatic Orb, which deals 3d8 damage.
While Guiding Bolt is a bit stronger than these others, its secondary benefit is also smaller.
This might be an encounter design issue
If your cleric is only ever casting healing spells and guiding bolt, that means the incentives in each fight are only based on direct damage to one enemy without terrain. One could imagine that save-based spells are better in areas with lots of cover, or AOE spells are better against a larger number of enemies.
Additionally, remember that a cleric only gets a certain number of 1st level spells per day. A ranger with 16 DEX and a heavy crossbow can deal 1d10+3 damage per hit, which averages 8.5 damage, and he can shoot essentially unlimited times per day. With multiattack, the ranger can deal double this damage. On the other hand, a cleric only ever gets 4 first-level spell slots. If you're fighting multiple encounters per day, as you should, those 4 spells will run out quickly, especially if the cleric has to do anything else. However, if you're only having one or two encounters a day, this built-in balance is broken.