As a collection of disparate clans, the Gruul have no single leader and no headquarters. But the Gruul respect strength and are willing to follow a strong individual who points them in a direction they want to go anyway. - Guildmaster's Guide to Ravnica, p.58
Gruul is difficult or impossible to actually get kicked out of in a technical sense, because it's a distributed collection of individuals and bands with no guildhall or central leadership with the authority to revoke your membership. How do you get thrown out of a group that's defined by anarchy and individualism?
You could get thrown out of a specific clan if you upset the clan leader (or upset somebody who convinces them to get rid of you), but realistically, nothing stops you from immediately joining another clan, unless the old clan's leader is strong enough and respected enough to make other local clans wary of allowing you in.
So it's more likely you've done something that angered your clan leader enough to declare you outcast, but not enough to get your head crushed (which might be a tricky tightrope); or you have made them mad enough for head-crushing and had to run for your life. You may need to just stay out of the Gruul-controlled areas until said leader gets replaced (or you find something worth trading for your sorry life). Borborgymos, the closest thing Gruul has to a guildmaster, is the obvious choice here, and would explain why all the clans hate you rather than just one, but any of them could potentially work.
Or it could be doctrinal. Where do your clerical powers come from? Usually in Ravnica, it's devotion to an ideal or philosophy. Maybe that source is at odds with Gruul philosophy in some way that makes your exile more self-imposed, at least until you can find a way to harmonize the two sides of yourself.
