HLG is Rec2020 it has to be resolved to Rec709 at some point unless only going to be seen on a TV that can resolve HDR10.
FCPX has a default custom lut plugin that can do just that in a Rec709 Library. Pop it on your footage and choose Rec2020 input and Rec709 output...to boot you can add a lut if you want to or just let the plug resolve the color space for you.