I have to GP5 files of the same song however 1 song is played at 70bpm and the other is 140bpm. How can I make the 140bpm file play at 70 but sound normal? Is there a way to half the note durations?
nope, you just have to change each one individually

and out of curiosity, why does it matter?
since 70 and 140 are essentially the same thing
Oh well some parts are played kind of differently, one version just sounds better. Am trying to pick bits from both to create a better sound/easier to play version.
Actually Ive heard theres a way to make a certain part of the song play at a different tempo in GP5. Anyone know how to do this?
Yeah I think you can half the note duration by selecting all the notes (Ctrl+A I guess) and then click on the minus (or plus, i forgot) symbol of your numpad.

However measures shouldn't be complete now, so you'll have some work to make them fit. maybe change the score signature from 4/4 to 2/4 for instance ...