The Magpies get their season under way against Southampton in eight days, and take on Ligue 1 side Brest on Saturday in their final warm-up game.
Sean Longstaff scored twice while Jacob Murphy and Anthony Gordon were also on target in United's penultimate friendly.
Girona finished third in La Liga last term and will compete in Europe's top club competition this season, but were outclassed by Eddie Howe's side.
Newcastle never looked back after taking an early lead through Longstaff, who finished off a great team move. He was involved in the build-up before Murphy turned neatly and advanced from his own half, then played the ball into space for Alexander Isak.
The Swede held the ball up in the right channel and brought Tino Livramento into play, with the full-back teeing up Longstaff - arriving late in the box - to finish cooly with his left foot.
It was 2-0 in the 13th minute, with Isak this time popping up on the left and digging out a cross, which Lewis Hall headed across goal for Murphy to fire home emphatically.
Moments later, Nick Pope made an important save, diving low to his right to get a strong hand to Yangel Herrera's shot - and within seconds, United were three goals up.
Isak was involved again, setting Gordon away on the break after Joelinton had won the ball in midfield. Gordon - who only returned to training on Tuesday after helping England reach the final of Euro 2024 - burst in on goal and slotted past Paulo Gazzaniga to mark his first appearance of pre-season with a goal.
All that was missing was a goal for Isak, and he had an opportunity to get one from the penalty spot. Jamie Miley picked him out on the left and Isak showed trademark close control, twisting and turning and jinking into the box before he was hauled down by Alejandro Frances. However, his spot-kick was saved by Gazzaniga at the Gallowgate End.
In the 25th minute, after more wonderful football from the Magpies, Longstaff set up Dan Burn, whose first-time left-footed effort was pushed away by the visiting goalkeeper.
But he could do little to prevent United from making it 4-0 in the 38th minute. He saved an initial effort from Isak, but Murphy recycled play and, following a scramble, Longstaff drilled home his second of the night.
Isak fired over at the beginning of the second half after positive play from teenager Trevan Sanusi, on at half time for Gordon in a planned change, then Murphy flashed wide in the 67th minute.
Five minutes later Isak was off target from a Murphy cut-back, then Murphy was denied by the legs of Gazzaniga late on, before substitute Ben Parkinson was also thwarted by the goalkeeper after great work from Sanusi and Isak.
Isak, finishing the game as strongly as he started it, was then prevented from making it 5-0 by the Argentina international goalkeeper.
But although there were no further goals in the second half, there was a Magpies debut for summer signing John Ruddy for the 31,958 crowd to cheer and the team were applauded loudly off the field after a hugely impressive showing.
Newcastle United: Nick Pope (John Ruddy 76), Joelinton, Anthony Gordon (Trevan Sanusi 46), Alexander Isak (Travis Hernes 89), Emil Krafth, Lewis Hall, Tino Livramento, Jacob Murphy, Dan Burn (c) (Ellis Stanton 89), Jamie Miley (Ben Parkinson 63), Sean Longstaff
Subs not used: Mark Gillespie, Harrison Ashby, Cathal Heffernan, Alfie Harrison, Anthony Munda, Leo Shahar
