TMRCA calculations give too wide of a range to be useful for genealogical research.
Matches with other surnames are very difficult to research. About all you can do is search the surname boards and internet with the matching name to see if anything has been uncovered.
A better approach would be to test a documented descendent of another son of the progenitor. If you are close matches, it would mean that the NPE, if any, occurred with or prior to the progenitor. There would be little chance of tracking it down, since you could not determine the time and place.
If you do not match, it would mean an NPE occurred in either of the two lines. You could then try to determined when it occurred by testing various branches. Even then, the only way to prove the biological father is to find a document.
FTDNA lists only 2 Gillbanks who have tested, so it is no surprise that there is no match.