The Panasonic GH5 is one of the few cameras that allows for high-quality internal video recording, but if you absolutely must have the best footage possible then you might want to invest in an Atomos Ninja Inferno. Atomos digital recorders are expensive accessories, but I have been very close to buying one multiple time for my Fujifilm X-T2. Of course, if you’re a serious videographer, then chances are you already own one of these useful tools.
Atomos Ninja Inferno: B&H Photo / Adorama
Panasonic GH5: B&H Photo / Amazon / Adorama
Panasonic GH5 Film Maker Kit: B&H Photo