If Anim Pakal is no longer on the battlefield when the triggered ability resolves, you'll still create Gnomes. Use the number of +1/+1 counters that were on it when it was last on the battlefield.

You choose which player, planeswalker, or battle the Gnome tokens are attacking. Each Gnome token can enter attacking a different player, planeswalker, or battle, and they don't need to be the same ones that the non-Gnome creatures are attacking.

Although the Gnome tokens enter the battlefield as attacking creatures, they were never declared as attacking creatures. Abilities that trigger whenever a creature attacks won't trigger when they enter the battlefield attacking.

Anim Pakal, Thousandth Moon doesn't have to be one of the non-Gnome creatures you attack with in order for its ability to trigger, but it can be.