I came across this in the Dictionary Corner feature of Radio Times:
For rural communities in the 17th century, 'gaffer' was a way to address a man of seniority. The word began as a shortened form of 'grandfather'.
So there is the solution to my dilemma. I can be a granddad without having to use the title. I think I shall like being called The Gaffer.
Of course, when she learns to talk, my granddaughter may have other ideas. At 2½ months she already has a lot to say for herself, loves splashing about in the bath or swimming pool and only cries when she's hungry or can't see what's going on around her. I have a feeling that she might end up being the gaffer.