Gnumeric cannot format range of cells if they already contain inconsistent formats
Cannot alter set the format for a range of cells if the range already contains cells with differing formats.
Say you have two rows of cells, both formatted to Number. But you have a few rows formatted to General. You decide to format all cells in both columns as General. So you select the columns and choose Format Columns. The Number tab will be greyed out, and its value shown as General. Since it is greyed out, it is not possible to set the format.
The result is that the cells must be formatted individually or subranges of like cells must be selected to change their formats. But since the underlying format might not be visible without opening the format dialog, it makes changing the format extremely onerous.