Among Oswal Jains, the surname/gotra Lunawat (also written Lunaawat, Lunavath, etc.) is traditionally understood to mean:

“Descendants of Luna (Lunaji)”

• Original ancestor (ādi‑purush): Luna
• Meaning of the name: “Lunawat” = the vaṃshaj (descendants) of Luna

So, the accepted ancestor of Lunawats is Luna (Lunaji), from whom this family name/gotra is derived.

You can read a bit more about Oswal surnames (including Lunawat) here