William Gowan/Gowin was not indentured but may have arrived in Bedford County to find work

In order for William Gowin to have been an indentured servant, he would have had to be free by 29 Aug 1754 when he was 21 years old. Two reasons: (1) The bounty of a child or an indentured servant or a slave accrued to their parent, master, or guardian. (2) Only a free male over 21 could make an assignment.

That argues strongly against an indenture. He was taxable to no one in 1748, 1748, and 1750. So the earliest he could have been indentured to Callaway would be 1751. He would not be free a mere three years later. 

The only way he could be indentured is if he was indentured to someone else and that someone then sold the contract to Callaway. That is so rare that it's practically non-existent. It's like suggesting that he was a lawyer from Boston who was married to Callaway's niece. It's just about as likely.

However, reaching 21 has nothing to do with indentures. But reaching 21 meant the end of apprenticeships and guardianships. Which were way, way more common.

Furthermore, indentured servants in 1750 were very rare to begin with. Virginia had discovered slavery, and slaves were doing what the indentured servants of an earlier generation used to do. Why would Callaway pay for an indenture when he could get a lifetime slave so easily? (Colonies that did not have slaves still had indentured servants, of course, but they had nearly disappeared from Virginia by that time.)

Furthermore, there is no evidence that Callaway kept any of the others that he paid tithes for as long as 7 years.

I would think it is most likely that Callaway's taxables were poor children under 21, orphans under 21, apprentices under 21, or overseers or other employees who were over 21.

Let’s examine one person who William Callaway also paid tithes for in 1749: Sherwood Gaddy.  In 1752 "Sherrod Gaddy" is listed as "overseer" on Thomas Mosely's list. "list" and "quarter" are codewords. That means that Mosely is non-resident, Gaddy is his resident overseer of the slaves taxed. One explanation is that once Gaddy turned 21 he hired himself out to Mosely as an overseer. That would place Gaddy’s birth about 1728 or 1729. This also fits with him being with Callaway 3 years prior to our William and thus 3 years older and likely around 18 years old when Gaddy was with Callaway.

Callaway is taxed for other white males each year. Gaddy in 1748, Brown in 1749 and 1750, then Gowen, Graves, and Simmmons in 1750. Possibly they are working in his store or warehouse. Or they may be field hands. Or they may each be at a different tract engaged in cutting down trees. We may never know. These men do not have families in the area. There are no other people with their surnames in the area. That suggests they are orphans or poor children or young men from somewhere else who are apprenticed to Callaway until they reach 21. 

Samuel Brown is not found in 1752 but is on the Bedford militia list in Sep 1758. William Gowen, Sherwood Gaddy, and William Simmons are on the same militia list. 

We can probably infer that William Gowin is not indentured, but was around 19 years old when William Callaway paid his tithes in 1752, and may have also been with him in 1751 for all we know (missing tithable list). This may help us with clues though.

Some, if not all, of these men appear to have been young when taxed to Callaway. Sherwood Gaddy seems to have been born in the late 1720s -- he died in 1803. William Gowin lived past 1800 and may have died around 1804. It was a rare man who lived past 70 in those days. Also, all of William Gowin's known children were born late 1750s or later. That suggests he did not marry until the mid-1750s or later.

Because there is no record of him in prior tithable lists, and because apprenticeships did not usually begin at age 18, it is not likely that he was an apprentice.  He was at least 19 years old when he was with Callaway.  There are also no other Gowin/Gowan/Gowen's in the area that could be potential relatives.  That means that he was either from a different geography than Lunenburg/Bedford County or that he was orphaned (a term used when no father is alive, regardless of whether mother is) prior to the date of the earliest availabe tithable list.  William Gowan/Gowin could also have been over 21 years old and working for Callaway. 

Conclusion: The men taxed to Callaway do not appear to have relatives in the area. Several indicators suggest that they were relatively young -- perhaps teenagers or early 20s. They may be teenagers apprenticed to Callaway, although unlikely. They may be poor children apprenticed to Callaway. They may be young men indentured to Callaway, although this is probably unlikely.  They may be "overseers" for Callaway although he didn't list them as such or working his tracts of land.