On Ubuntu 18.04, gmplayer was failing to start, with error message: "Error in skin config file at line 6: PNG read error in /usr/share/mplayer/skins/default/main". This is apparently a bug that has been around for at least 5 years and has never been fixed (even in Ubuntu 20.04). Thanks to a post on AskUbuntu it can be fixed by doing:

cd /usr/share/mplayer/skins/default
for FILE in *.png
do
       sudo convert $FILE -define png:format=png24 $FILE
done