I wasn't about to go through the precautionary backup and reinstall process for something I don't use (I don't even get an increase in frame rate when I do lightmap), but I used the Steam app to verify the integrity of the game cache and 2 files apparently needed to be fixed. It gave no indication that it was fixing them, but I waited a few minutes and launched GG, threw a couple buildings in and ran the lightmapper. It didn't do the whole of the map this time, but like I said above, it doesn't do it every time for me anyway. So that's where that is anyway!
MAME Cab PC: i7 4770@3.4Ghz (passmark 9945), 12GB RAM, Win 8.1/64, GeForce GTX645 (passmark 1898); Shiny new laptop: i7 4800MQ@2.7Ghz (passmark 8586), 16GB RAM, Win 8.1/64, GeForce GTX870M (passmark 3598); Old laptop: i5@2.3Ghz, 8GB RAM, Win 7/64, Intel 3000 graphics