The form gentaculavit implies it’s a 3rd person singular perfect indicative of a regular 1st conjugation Late Latin verb gentaculare “to eat breakfast”. Knowing that, it’s clearly derived from this Late Latin noun:

https://en.m.wiktionary.org/wiki/jentaculum#Latin

From Wiktionary:

gentaculum “breakfast” (alternate spelling of ientaculum/jentaculum). That form ultimately comes from the adjective ieiunus (alternate spelling jejunus) “fasting”.

The adjective is a standard word in Classical Latin, but the noun and verb above are not. They are probably later developments or Vulgar Latin (spoken by the common people) that was not preserved in Classical texts. The standard word for breakfast in Classical Latin would be prandium.

It appears to be related to ientaculum, meaning breakfast.

https://en.m.wiktionary.org/wiki/ientaculum#Latin