Certain characters can safe jump 3 frame DP’s because their jumping attacks have enough range and strong enough hit-boxes to protect themselves against DP’s.
Another class of safe-jump is an ambiguous cross-up that forces reversals to go the wrong way. It’s safe because the only way to get the reversal to come out the right way is to delay it till 1f after you’ve woken up (which technically means it’s not a reversal any more) but that essentially adds 1f of startup to the move, so 3f reversals become safe-jumpable. A weaker version of this tactic relies on unsafe jump timing or gives the opponent more room to delay their reversals (which means it’s not a safe jump) but it’s still hard to counter eg.
Note that (at least in Vanilla) there is a bug in the game that will cause your setups to fail even if you time it perfectly (I’m guessing maybe 1 in 50 times). Sometimes it’s caused by the direction the opponent is holding as they wake up, other times it just seems completely random. In the previous setup vs Ken, sometimes that j.hp doesn’t cross-up so Ken is able to reversal DP because of this.
AFAIK as Gief goes, I’m not sure what his safe jump setups are. You might get some help on the Gief forums.