Garp, Woop Slap….. oh, please! Are you guys serious?!
The only person Oda could refer to with "that person" should be Shanks and no one else. He's the only one who we know having connections with Makino. And it's not a bit impossible that Shanks was visiting Fuusha in those two years! He did it in the past, when he was associated with Luffy too. The new world may be far away, but if he sails through the calm belt it's all the more possible. And Shanks always seems to have had some sort of connection to that Village (see Volume 1 where it's stated, that the Red Hair pirates often visited Fuusha. And Shanks said, when leaving Luffy, that THIS TIME, he would not be coming soon again. Which means he WOULD come again, just not soon).
And btw it couldn't be Garp in the first place. The Monkey D. guys have/had all black hair and Makinos Child has bright hair.... And in generall, Garp (not to mention Woop Slap) could be Makinos grandfather! I don't trust Oda (as a father) to make such age difference connections in his manga....
For me it's clearly Shanks. The fact that Oda says no name imo is just because he wouldn't reveal such an important information (could be important at the end of OP maybe, who knows..) directly in the SBS. So he leaves it with hinting it..