That seems like the most likely explanation, although there are a few parts to this that should be dissected. Spoiler tags for Anime only watchers.
1.Based off recent chapters in the Manga, Big Mom has heard of the "hunts" of innocents that the Celestial Dragons have performed, even if she dismissed them as rumors. Dragon, a low ranking marine grunt in the current flashback, is aware of the "hunts" as is the rest of his platoon (They tried to interfere and Dragon's whole platoon got executed for this). Considering a pirate and a low-ranking marine know of the "hunts", there isn't much room for doubt that Garp (a Vice Admiral) indeed knows of them.
2.The next few chapters will elaborate on this more, but Chapter 957 gave us a lot. Garp dislikes the Celestial Dragons and refused the admiral position due to not wanting to be placed under the Celestial Dragon's direct command. Sengoku goes further with this though. The insubordination of refusing to do their bidding actually is implied to put his life in danger, with only his accomplishments/stature preventing this. If Garp goes any further than just ignoring their wishes, he would probably get eliminated. Also, only tangentially related, but it is very much in doubt that Garp actually protected the "Celestial Dragon's Slaves" as Sengoku implied, considering the "hunt" that was going on. https://gamefaqs.gamespot.com/a/forum/5/525d4ce9.jpg
3.Just an irritating thing that I've noticed, but Oda always has the "good marines", of which Garp is implied to be, absent when the Celestial Dragons are being...Celestial Dragons. I mean, Garp escorted Shirahoshi and her family to/from the Levely. Not too long after escorting her to the event, he is missing both when Saint Charlos tries to enslave Shirahoshi and CP9 backs up this attempt to enslave her as well as when later he is MIA to prevent presumably Lucci from abducting Vivi not too long after Vivi wished Garp was protecting her instead.
I don't believe that having Garp not realize/see what is happening at God Valley is realistic though...and considering they are on speaking terms even in the present day, Dragon at the very least must have definitely told him what happened with the "hunt". So yeah, hopefully Oda does show Garp having to face this. It feels...cowardly not to.
4. Not as relevant, but Garp right now is Blackbeard's prisoner. Considering the God Valley incident, both were there and that may be relevant...although Blackbeard just sees him as a bargaining chip. That may change of course.
Yeah, I meant way before that, more like when Garp first joined the Marines. Im guessing he didnt know all that when he was a rookie like Koby and probably advanced some ranks before finding out how heinous the WG was.
But hes Kongs son so who knows? It could certainly have been that he was raised witnessing all that shit.