Parashas Shemos
וַתֵּ֣לֶד בֵּ֔ן וַיִּקְרָ֥א אֶת־שְׁמ֖וֹ גֵּרְשֹׁ֑ם כִּ֣י אָמַ֔ר גֵּ֣ר הָיִ֔יתִי בְּאֶ֖רֶץ נָכְרִיָּֽה׃
And she bore a son, and he called his name Gershom, for he said: I have been a stranger in a foreign land.
Torah Temimah points to TT on 2:22 (verified).
גרשם — מכאן דשם גרשם לעצמו ושם גרשון לעצמו, ונ״מ לענין שמות בגיטין.
English: “Gershom” — from here [we learn] that the name Gershom is distinct from the name Gershon, with practical consequence for names in gittin.
Custom
Chazal note that Gershom and Gershon are distinct names, a distinction that matters for the exactitude of names in a get.