Eh. I am pretty sure Garp likes things being stable and peaceful, which the WG does accomplish on the surface. Which could be a huge plot hole because he should have seen enough of the WG's darker shit that the WG would clearly fall out of his moral compass and not warrant his loyalty. But then again he might just be loyal to the marines, who he might view as a separate peacekeeping force but... and we could go in circles about this all day.
The reality is that Garp (and the marines in general) could be an after thought writing wise. But hey, most modern day soldiers are also good people who do shady shit and are told it’s for a greater cause when they are just working for objectively evil people. So maybe that is Oda's grander narrative about the marines as well.