When libgig support is enabled, you have to select a .gig file twice in QSampler's channel dialog in order to be able to see all instruments of that .gig file in the dropdown list and thus to actually be able to load an instrument. I think this is an important bug to fix! I haven't really looked into this issue yet. Do you know the reason for this problem? Is it an issue with libgig? At least qsampler doesn't block.
Fixed with QSampler 0.0.9.42.