Gridlength.Auto makes sense: it will lay out the row or column automatically in the available space.
new GridLength(100, GridUnitType.Absolute) makes sense: you assign a certain number of pixels, in this case 100, for that row or column.
But how about if I want to lay out my grid in percentage terms? 20% for Column 1, 34% for Column 2, 28% for Column 3 and Auto, or 'the rest' for Column 4? There doesn't seem to be an option for this.
GridUnitType.Star isn't explained well anywhere, but in any case it doesn't lay things out in percentage terms.