This flashback Garp, and maybe even his strongest attack back then, is comparable to OKG at best. The blows he used against Xebec seem, from the context, to be more like Gear 3-level strikes, which scale much lower than Gear 4 in terms of raw power.
The flashback basically only served to show how insanely strong Kaidou really is — he would eat that God Valley Garp for breakfast.