Microsoft removed and re-issued the Get Windows 10 from Windows Update June 8th, 2015, Revised June 9th, 2015
June 9th, Patch Tuesday As of 12:00 Noon KB2952664 is back, run Windows Update to install this update and complete the GWX process. Reboot your computer after the update has been installed.
Get Windows 10 (GWX) Windows Update not listed I just spent nearly a whole week documenting how to get that GWX icon to display more quickly on the taskbar and wanted to do one last verification that it worked only to find that after 3 tries at installing Windows Update one of the 2 key updates (KB3035583 and KB2952664 for Windows 7 with SP1 installed) was no longer being listed in Windows Update. For Windows 7 it’s KB2952664 that missing. For Windows 8.1 users some have reported they can find KB3035583 and it’s possible that KB2976978 may also been pulled.
No idea why except for possible flaws or too many complaints that the GWX icon is in effect Adware.