Have you checked to see if she is on the 1830 or 1840 census?
Also in my journey of the Gross family I had a Dr. Janet Gross in Australia contact me. I believe her to the expert on the family. I can email her to see if she has any information on Josephine Gross.
You may also want to jiggle the birth dates by going back or forward to see if a leaf appears.
Have you gone back to Isaac Gross and the families arrival in Boston yet in about 1635?
Let me know what you find first by jiggling the dates. If no luck we can email Dr. Gross.