John was born about 1770-75 and shows up in the 1820 and 1830 censuses. Was he born in PA or VA? Who are his children? Who are his parents? Is Thomas Gregg b. ca. 1811 who married Jane Trowbridge one of his sons?

Some of these Gregg/Grogg/Gragg lines trace back to a family in Delaware.

http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&d...

There is evidence that he is a son of Henry who was son of Thomas. They were in VA for quite awhile, but the names are also consistant with the family from Delaware via Ireland and Scotland.

When did John come to was is now Pendleton County?

I’m not related to any member of this family.

This is your problem.
Without "sources" you cannot establish relationships.

Facts you should know about the early census records –

All census records [1790 – 1840] prior to the 1850 census ONLY listed the head of household; whether male or female.

NO specific age was stated for any family member
NO place of birth was stated – city, state, or country
NO city, town, or village is stated – only the county; however some census takers listed the township
NO street address was stated
NO marital status was stated – single, married, widowed, or divorced
NO family relationship was stated – brother, sister, cousin, son, daughter, wife, inlaw, etc…
NO occupation was stated
NO parental birthplaces are stated
NO race was stated [but assume “white”]

1850, 1860 & 1870 census records do not show family relationships, marital status or parental birthplaces.

Step children are not enumerated as “step” children
Adopted children are not enumerated as “adopted”
Grand children are not enumerated as “grand children”
Orphaned children were not enumerated as "orphan"

1850 is the 1st census that shows all family members

1880 is the 1st census that shows parental birthplaces and family relationships +++

Immigration data was not asked on census records until the 1900 census.+++