You either backport it yourself or try to install the version in testing.
Package gnome-mplayer
* squeeze (testing) (video): A GTK+ interface for MPlayer
0.9.9.2-1: amd64 armel hppa i386 ia64 kfreebsd-amd64 kfreebsd-i386 mips mipsel powerpc s390 sparc
* sid (unstable) (video): A GTK+ interface for MPlayer
0.9.9.2-1: alpha amd64 armel avr32 hppa hurd-i386 i386 ia64 kfreebsd-amd64 kfreebsd-i386 mips mipsel powerpc s390 sparc
0.9.6-1 [debports]: m68k
http://www.debian.org/distrib/packages