Hi,
Grigalius is lithuanianised christian name Gregor(greek). In this case Grigalius is shortened to Grigas and then diminitive suffix -ul is added. There are a lot of surnames with root Grig- in my dictionary. Hope it helps.
Ps.: Grigula most popular in Varniai, Kražiai, Rietavas, Tveriai and Klaipėda districts. The surname is probably of samogitian descent because it is almost nonexistent east of Samogitia (West Lithuania).
I just saw your other thread. If you have time Kražiai Church records are available for you
http://www.epaveldas.lt (just copy-paste Kražių in Antraštė form and uncheck bibliotekos and muziejai). Unfortunately the written language is Latin and russian (from about 1800).