The gift is halal for you. However, it is necessary for your mother to do taubah and istighfar for the negligence she committed in paying zakah and she should pay zakah of those years after calculation. You may also pay the zakah of your mother with her permission. You also will have to pay zakah of the gold since it came into your possession.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband