Opticallimits reviewed the Sony FE 100-400mm GM and concluded:
The Sony FE 100-400mm f/4.5-5.6 GM OSS is interesting for users who prefer a more compact size, lower weight and higher versatility compared to the current breed of 500mm/600mm zoom lenses. However, it isn’t as superlative as the high price tag and the GM label may suggest.
I also do think the 100-400mm GM is one of those lenses that are in need for an update!