I have had mixed results with different species of Plecos and Gars. The best advice I can give you is if you add them be very carfull and be prepared to remove the Pleco at the first signs of them going after the Gar. I am very lucky and have a local petshop take many Plecos from me that did not work out and give me good trade deals on a different species, all arrangements were made in advance and cleared with management before the first Pleco went into the tank.
I have had commons from 5" to 8" go after my Gars but around 9" and above they left them alone completly, so size may or may play a factor in this. Gibby Plecos of any size that were not small enough to be eaten, and I have had over a dozen in the tank over the past 10 years ( got too a trade deal on a large Gibby to keep them ) have never been an issue for me with the Gars as well as Royals. L-114, L-014, were removed after a few days, I can tell you right now these WILL go after the Gar once they settle into the tank.