A descendant of Gorhendad the founder of Buckland, and in turn an important ancestor of the Brandybuck family, Gormadoc is listed at the head of the genealogical table given in The Lord of the Rings. He married Malva Headstrong, and had three children, Madoc, Sadoc and Marroc. Madoc the eldest succeeded his father as Master of Buckland, and passed the line on through the generations to the later Masters, eventually including Meriadoc. Gormadoc was also the great-great-great-grandfather of Frodo Baggins (through Frodo's mother Primula Brandybuck).
Gormadoc's nickname 'Deepdelver' seems to have its origins in Tolkien's first drafts of the Brandybuck family tree. At first, these placed Gorhendad, not Gormadoc, at the head of the tree, and Gorhendad was not only Buckland's founder, but also the delver of Brandy Hall in Buck Hill. Gorhendad was ultimately removed from this place and moved further back into history, but the idea of his being a 'Deepdelver' remained attached to the head of the family tree, and so passed on to his replacement Gormadoc. In this new context, it is presumably meant to suggest that Gormadoc expanded the Hall and delved deeper into the Hill.
Notes
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The names of members of the Brandybuck family tend to be influenced by Old Welsh, and 'Gormadoc' is no exception. Madoc means 'fortunate', but the meaning of the 'Gor-' prefix is less certain. It perhaps relates to guor or guar meaning 'over', and if so in context Gormadoc's name would mean something like 'very fortunate'.