Just click GMPlayer icon itself to start it in GUI mode (it's a project icon and it's done on purpose). Alternatively, one can also enable GUI tooltype in MPlayer icon.
@aurora
If video still lags a bit behind with loopfilter disabled, then you can't do much more. Keep in mind it's much slower on OSX or Linux, but 720p can be a bit too much for a 1.5GHz G4 if bitrate is a bit too high.
