Is there some of the GPS code missing? The declarations and includes before setup()?
Everything before setup() wants to be in your new sketch. Everything in BOTH setup() wants to be in the setup() of your new sketch - but you need to sit down and make sure that you aren't trying to use any pins for more than one task, unless that is what you want! I haven't been through the code thoroughly but as I have that screen combo I suspect there will be conflict, and if there is you will need to move Serial1 to other pins.
And then the difficult bit - read the gps, decode the message and output to lcd!
(Whoops - I forgot it was a Mega clone - but the basics are still right!)