A simple add-on accessory provides GPS tagging to the 1D X and 5DIII. I highly doubt that many people opted for the 6D over the 5DIII for built-in GPS, rather, the overwhelming majority made that choice because the 6D costs much less.
It would have been nice if this GPS was included in the upper end bodies.
Maybe there are technical reasons why it wasn't.
On the surface, it seems that there is significant intentional fragmentation within the Canon lineup with features that should be common within the line. This could be one of those features and WiFi is another that comes to mind.
I would have far more interest in either of those features than the movie capacity that is included in the camera bodies.
Choosing a camera body for GPS features means I give up many possible AF points that I find useful for off-center composition. There is a huge price gulf between the 6D, 5DII/III and the 1DX or future rumored 1DXs. I would suspect that the number of off-center picture composers outnumber the camera users that really and truly need all those AF points for action shooting.
Going from the 5DIII to the 6D to pick up internal GPS features seems very unlikely (excluding other revolutionary changes, 5DIII to "5DIV" for internal GPS is also unlikely- at least for me). So much is given up in other areas. I can understan product differentiation through sensor size, megapixels, ISO range, viewfinder, AF points/speed, metering and so forth. If internal GPS and/or WiFi was available across the 6D, 5D and 1DX lines, would this feature really cannibalize sales?
From my perspective as a product user, keeping GPS and/or WiFi out of the mix makes no sense- I realize that I have no knowledge of legit technical issues or product development timelines that may have excluded internal GPS or WiFi features. My phones have had GPS and WiFi for a long time without an external add-on, so the exclusion of this tech from an expensive camera is especially baffling.
I do not own the GP-E2, nor have I seen one in person. Based on a quick search of online reviews, the device is either connected to the hotshoe or to the camera via a cable. Either seems to pose potential pitfalls compared to having the GPS internal to the camera body: no flash or snag the cable on something. There also seems to be some issues with the type of files generated by the Canon GPS unit, but this may have been "fixed" with later versions of 3rd party programs such as Lightroom.
I would be interested in feedback concerning the GP-E2.