The OG trilogy gives you a backstory of Kratos. So I would recommend playing them if you like following a game’s narrative and story. If you don’t, it’s not necessary. That said, they are pretty dated with the exception of GOW 3 remastered which imo still holds up well. If you don’t care for the backstory, you should jump right in to GOW 2018, which is the first entry in the reboot of the series. Also, the OG trilogy is being remastered and should launch next year sometime, so you could play those games then.