I'm unable to open JPEG files as a source image in version 3.6.6 of the G'MIC-Qt standalone interface for Windows. Apparently since version 3.6.5 the Qt imageformats directory is AWOL. Weird thing is that by for example using the Import Data filter under Various JPEG files can be opened and are read correctly.
Copying the imageformats directory from version 3.6.4 into the 3.6.6 base directory restores JPEG support.