I have a bank loan judgement against a property that is not a primary residence. Can I gift this property to a child without paying the judgement?Can he sell the property if he assumes the mortgage?

No it can not be "sold" that is what a gift transfer is, a sell without any money.

The liens will have to be paid for you to transfer it, he can take a loan out and pay off the judgement and the mortgage.

Property cannot be transferred to another party until all liens are satisfied. Otherwise the transfer would not be legal and might be considered fraud.