HLG is a HDR standard that provides backwards compatibility with non-HDR monitors while achieving a wider color gamut for HDR monitors. Because HLG EOTF is similar to standard gamma functions, HLG streams look correct when played on SDR monitors. Meanwhile PQ EOTF streams will not be displayed properly and usually look washed out when played on a SDR monitor because the gamma and PQ curves are different. Unlike HDR10, HLG does not require any extra metadata with the video data. HLG streams consist of the HLG transfer characteristics/EOTF and BT2020 color primaries and BT2020 color matrix.
For information on how HLG is processed by the software stack, see H.264/H.265 Video Codec Unit LogiCORE IP Product Guide (PG252).