If you activate this effect, you must send 1 Gemini monster from your Deck and draw 1 card. You cannot choose to just do only one part of the effect.[1]
If you do not have a Gemini monster in your Deck, you cannot activate this effect.[1]
If your Deck contains 1 or fewer cards, even if your remaining card is a Gemini monster you cannot activate this effect.[1]
OCG Rulings
If you choose to activate the effect of "Featherizer", then you must both send a Gemini Monster to the Graveyard and draw one card. You cannot choose to only perform one.[2]