Reporting observation of a bird outside its seasonal presence
It is NOT a Great Grey Shrike (Lanius excubitor).

It is a young Lanius minor Lesser Grey Shrike (which breeds in Bulgaria).
I say young (born the same year) because of the white tips of the feathers (they are new) and not so extended black mask (which probably induced you into the ID mistake).

I attach a picture of young Lesser Grey Shrike for comparison.

Generally you can report your observation on ebird.
There is a team checking the unusual observations (although for Europe at least many mistakes are not picked up).