It would appear that the only salient difference in proprietary-ness is that KML was developed by a single company (albeit with input from others), whereas GML was developed by a standards committee. The salient difference in the marketplace is that KML is usable and hand-editable, whereas GML is rather too complex for use without tools. In contrast to what one might expect, the standards committee developed format requires tools to create, whereas the one developed by Keyhole does not.
Strong words against the GML camp, but some of it might be deserved. Personally I’ve not run into too many people complaining about KML being proprietary, but I’ve seen people bring it up on different blogs. As a GIS professional I don’t really care about which formats have OGC standard labels on them as most of our clients use one of 3 formats for data interchange; e00, shapefile or personal geodatabase, all ESRI formats. I don’t see the day anytime soon where KML will get added to that list, but if ESRI continues to integrate KML support into their products and continues to ignore GML it could happen.
I got hammered a months ago about complaining that much of these open formats aren’t supported in ArcCatalog and saying it was up to the open source community integrate into ArcGIS products, rather than ESRI. I think at this point with ESRI OGC support usually limited to an ArcGIS extension, someone needs to step up and write some tools for GIS professionals to integrate OGC support into their ESRI workflows because ESRI doesn’t seem to think it is a priority. There are tons of open source tools that support both OGC and proprietary ESRI standards and formats, but you have to pay money on top of ArcGIS to get such support in ArcGIS. Many look at this as ESRI’s fault and maybe it is, but given all the KML export/import tools being developed for ArcGIS, you’d think someone would take on the challenge and integrate all the GDAL tools into a nice toolbar or toolbox.