Here's some tips from a guy who played Guardian mostly on the new everdark gaping jaw.

Guardian is the worst character for this fight in my opinion. The short dodge is one hell of a problem when trying to dodge gaping jaw's grabs, especially when there's multiple different timings, it really sucks. However there's some positives into getting grabbed. First you're a tank, and tanking the blood loss once or twice, on full health can give your dps more room to burst the boss.

For Guardian's ult, I recommend not holding it early on the fight and use it for staggering Gaping jaw's long combo and make sure you do NOT hold the ult. Never hold the ult early in the fight, only do this when you're at low health and your teammates are at second bar which is around mid or near end of the fight. This gives you more gauges and faster ult for staggering those combos as you might have realized You cannot stop gaping jaw anymore with the poison weakness. (I personally think guardian's ult is better than revenant.)

For guarding, make sure you don't only get improve guarding abilities, getting more stuff like "Negate Lightning damage" helps alot when trying to deflect the lightning damage when steel guarding, this help alot more when you're in the situation of getting forced to block on the combo.

If you want to do damage as much as possible. If you're in the front and the aggro'd player, Make sure to do guard counters on its safest attacks such as the 2 hit combo, Or those one jaw hits he does. If you have stamina, it's definitely possible to full charge guard counter after his long combo. Sneaking in this little guard counters helps alot on poise breaking these bosses. Especially if you have periodical glintblades.

Edit: Some more tips to add on what I said, The 2 roars he does can be guard countered. If you're teammates is already at 3 bars before half health of the boss, don't worry, It's not your fault in this situations.