When you gift a minifig, the pieces they are wearing are also gifted from your inventory. There are some promotional pieces that cannot be gifted individually or on a minifig. These are pieces that you can only get from a LEGO Tower promotion and cannot be obtained in the game otherwise. If you try gifting a minifig wearing one or more of these promotional pieces, the "Send to friend" option will be greyed out until you remove those piece(s). If you want to gift a minifig but not the limited quality piece(s) they are wearing, there is an infinity button in the "Change Pieces" menu that will randomly dress the minifig in pieces that you have an unlimited quantity of.