BB maybe. But, oda already revealed that Rocks was BB's dad in this flashback. If oda was gonna do gv as BB's flashback, I think he would have put off that into till later.
And, I don't think BB actually saw Rocks at all. He was a toddler. I think BB's flashback would be him growing up with his mom and two sisters. Them dying and him becoming an orphan. Getting into wbp, and slowly growing his power and connections to the of bbp members etc.
Not gv.
Giving the full gv flashback rn as part of Harald flashback makes perfect sense.