The easiest way is to right-click on the piece that you want to move. That will use the lower dice first.

Alternatively you can drag the piece to any of the places that it can go. In the Options dialogue, on the Display tab, there's an checkbox called "Show target help when dragging a chequer". If this is ticked then, when you start to drag a piece, all the possible destinations will be highlighted with a green ckecker. You can drag the piece to any of those points and drop it. That's sometimes useful if you want to play all four moves of a double without having to click the piece around the board but can also be used to do the lower dice first.


So, in essence, it's gaining from someone who hasn't learned how to use the program properly, either by reading the documentation http://www.gnubg.org/documentation/doku.php?id=playing_a_game_or_match#18 or by asking for help. You can hardly call that cheating!

Still, you've asked now, so all is good. You might like to read the rest of the manual and see what other tips you're missing out on.
http://www.gnubg.org/documentation/doku.php?id=gnu_backgammon_manual