Okay, then it must be
this right? I can't think of any other instances of him discussing the Angel law. In these pages it's stated that the GP enforces Angel law and seems to more or less serve as the judge for them, with him deciding when it's 'too far' and the Angel needs to be erased. However we are also shown that the GP found a loophole for Merus to save him so either he can't fully go against the law or he simply created the loophole to 'technically' follow his own law.
From there we're going to need to concretely argue that Angel law is integral to reality and isn't simply just akin to human law. Is the Grand Priest actually controlling the limits of a divine law? Or is it just a ruling made up by free will? Well
it seems the erasure process is natural, given Beerus believes they would need to explain to the GP what happened, but the GP does allude to ending Merus himself prior to Whis making his case.
So:
- Angelic law is integral to existence, it's not just made-up like human law and Merus is erased automatically for violating it
- Yet the Grand Priest mentions 'ending Merus's tenure' himself, meaning either he would kill Merus himself or he chooses when the law takes effect (defining the law's limits)
- The only reason Merus isn't erased there is because Whis provided a convincing argument to give him more leeway (greater limits)
There's no reason for the Grand Priest to step in and end Merus's tenure himself if the law is integral to existence like it's shown. Not unless the implication is that the Grand Priest has some level of control over that law, a law shown to be part of their very existence and taking effect automatically.
In short it does seem like the Grand Priest is actively controlling the law, meaning there is some argument for GP having Law Manipulation.