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.

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.

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.