The context menu in the cell grid area appears when the pointer is over the cell grid area. This menu applies to the cells that have been selected, not necessarily the cell underneath the mouse pointer.
The grid context menu merely provides an extra way to access Gnumeric commands. The Cut, Copy, Paste, Paste Special, Delete Cells..., Clear Contents, Remove Comment, Edit Hyperlink, and Remove Link commands are taken from the Edit menu and its submenus and are explained in Section 4.2.3 ― The Edit Menu. The Insert Cells... and Add Comment menu items are explained in the section on the Insert menu in Section 4.2.5 ― The Insert Menu. The Format Cells..., Conditional Formatting... items as well as the items on the Cell, Column, and Row submenus are explained in the manual section on the Format menu in Section 4.2.6 ― The Format Menu.