Radak: It was Livni. Below (23:10), it says that Yachas was Ben Shim'i! Two different men were called Yachas. There (7), Livni's ¹ son is called Yechi'el. This is like Yachas; he was called both of them. This occurs many times!

Malbim: Verse 28 says that it was Gershom! There are omissions both here and below. The Sefer relies on the reader to fill in the omissions in both places. The chain descending from Livni was Yachas, Shim'i, Zimah, Eisan, Yo'av. Here it could not omit Yachas, and list Shim'i after Livni, lest he be confused with Shim'i ben Livni from Merari (14).

'Rashi': Also in Parshas Pinchas they are omitted, and also the families of Amram, Chevron and Uzi'el are omitted. This is because it mentions singers ¹ . Bnei Shim'i are counted when David counted them (23:10).