Grabbing Power - this measures how much damage the victim of a grab (or grab related move) receives when he or she escapes a grab from another character hitting the foe that has you grabbed. For example, let's use the characters Fox, Sheik and Marth. Sheik grabs Marth, then Fox hits Sheik with an uncharged down smash. Sheik receives 14% from the smash, but Marth receives a certain amount (usually 2% and 6%) of damage from the release of the grab from someone hitting Sheik, the grabber. In this case, Marth loses 6% because Sheik has a grabbing power of 6%. I also want to say that usually a victim of a grab receives 3% damage if he/she escapes a throw from someone else hitting the foe that grabbed you. 

10% - DK's carry move(if the victim escapes DK's hand's while he's carrying the other character, then BOTH characters receive 10%)

6% - Bowser / Ice Climbers / Jigglypuff / Kirby / Peach / Sheik / and Zelda's grab. Also, it's from Ganondorf and Captain Falcon's Up B move if another character hits Ganondorf / Captain Falcon and helps you escape while the move is still in effect.

5% - Kirby's B move(when someone attacks Kirby when you're in his mouth, you receive 5% or the % done from the attack that freed you). It's also from Boswer's forward B (Koopa Claw) and Mewtwo's forward B (Confusion, when character is still spinning).

4% - Yoshi's B move when you're in Yoshi's mouth (not from Yoshi's grab with Z).

3% - getting freed from most throws.

2% - everyone else's grab power is 2%.

I don't know about Giga Bowser, but who really cares.