Do they garnish gross or net wages?

They garnish on your net... and for example, if you have a retirement account going, then they will take out retirement FIRST, and then the garnishment amount. Sometimes the garnishment can be a flat amount, or based upon a percentage of what you earned.

The Federal government prohibits creditors from collecting more than 25% of your gross pay, except in cases of familial support. Your garnishment deduction may be calculated based on your gross earnings.