ガシッと	15971
がしっと	12251
がっしと	1961
ガッシと	1090
----
がっしと掴む	59
がっしりと掴む	215
がしっと掴む	280

I think it's a variant member of the がっしり(と)/がっしと clan. The meanings and usage align.
Probably the cleanest thing is to add it to the ガシッと/がしっと entry. I'll do that and keep it open in case anyone wants to comment/expand/challenge.

https://thesaurus.weblio.jp/content/がっしと掴む

https://jpdb.io/vocabulary/1284440/がっする?expand=p#examples_plus_2466231291

I suggest this a bit tentatively, because I don't find it in other online dictionaries, though the weblio thesaurus has an entry for がっしと掴む which it defines as ものを手などでしっかりと持つこと. I am guessing from that plus the other examples I see that this is a variant of がしっと and I have copied that entry's definition.

I originally found this because jpdb.io's corpus has a fair number of these, which it currently mis-parses and files under がっする because of the lack of a JMdict entry for がっし.

Google searches for "がっしと掴む" seem to get a reasonable number of hits. Google book search also finds a few hits in published books. Still, maybe this is all just typos ?? I leave it up to your judgement whether this ought to get an entry.

You should check the tags also, as I think I've probably got those wrong (attempting to copy from がしっと).