An onomatopoeic/mimetic adverb meaning robustly/solidly/sturdily — describes a solid build or firm construction.
彼はがっしりした体格で、スポーツが得意だ。
He has a sturdy build and is good at sports.
この家具はがっしり作られていて長持ちしそうだ。
This furniture is solidly built and seems likely to last a long time.
ドアノブをがっしり握って開けた。
I grabbed the doorknob firmly and opened it.