Complex sentence: Unfortunately, many users did not understand the difference between "free of charge" and a "free gift", where difference is that a free gift is given without any expectations of future compensation, while something provided free of charge expects something in return.
Split into simpler sentences: Unfortunately, many users did not understand the difference between "free of charge" and a "free gift". The difference is that a free gift is given without any expectations of future compensation, but something provided free of charge expects something in return.