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Male Gustavus Rockwell Click to view Gustavus Rockwell in the family tree

Gustavus was born after 1750.1   Gustavus' father was Samuel Rockwell and his mother was Mabel Mehitable Loomis.  His paternal grandparents were Thomas Grant and Elizabeth Rockwell; his maternal grandparents were Moses Loomis and Elizabeth Bidwell. He had two brothers and two sisters, and also a sibling of unknown gender, named Thomas, Sylvester, Tryphena, Elizabeth and Wereham.  He was the oldest of the six children.1 

1 phelps.FTW @ Tree #0105
Further sources/citations:
World Family Tree Vol. 3, Ed. 1 @ Tree #3669, 3669.ftw