My understanding is that with current technology (WAAS - so only in the USA), GPS can't be relied upon to be more accurate than +- 7.6 meters at least 95% of the time (that is from the WAAS specification) horizontally and vertically. If you have a large commercial runway 45m wide (which is likely the case if there is an ILS available anyway) then you are still going to hit it. However without some way of determining height (ie a radar altimeter) then landing with your head down simply isn't safe (your guess when to flare is as good as mine!).
I believe WAAS allows GPS to be used for Cat I approaches in the US - but most of us on this forum aren't in the US.
Without the additional checking that WAAS provides you need to have some other way of verifying that your GPS is accurate.
Maybe once
EGNOS is available it will be different.
Until then GPS can only be a secondary navigational source in Europe.