Read every card...on Gambler it specifically states 'shuffle your hand into your deck'. Since your 'hand' can be a 'hand' of zero cards you could shuffle if you had no cards in your hand in this case. However, cards that say trade a card in your hand for one in your deck, and you don't have one to trade; you can't shuffle in that case. Lass says 'shuffle all Trainer cards in your hand back into your deck. If you have no trainers, you can't shuffle your deck.