Not used to seeing a juvie with such rough plumage, but good points on the lack of black cap and soft gape (this later point I especially should have particularly caught on...).
No chance of it being a Boat-tailed Grackle. It is for sure a Gray Catbird with the slate gray plumage, and you can see a tiny hint of rust-coloured undertail coverts which is diagnostic of the Gray Catbird.