Visa/MC are 'open loop' cards meaning they can be used anywhere and Target GC's are 'closed loop' meaning they are only good at Target. Typically, even if a store will allow you to purchase a GC with their store GC (ie, using Target to purchase Disney), they will not allow you to use their GC to purchase an open loop card.
I've never tried it at Target to know if it is hard coded in the registers to deny it or not but I'd be really surprised if it worked.