Complex sentence: License compatibility between a copyleft license and another license is often only a one-way compatibility, making the copyleft license (GPL, and most other copyleft licenses) incompatible with proprietary commercial licenses, as well as with many non-proprietary licenses.
Split into simpler sentences: License compatibility between a copyleft license and another license is often only a one-way compatibility. This characteristic makes the copyleft GPL (and most other copyleft licenses) incompatible with proprietary commercial licenses and also many non-proprietary licenses.