When you come upon Mexicans eating — remembering to say provecho, of course — they may offer to share their food, even if they know you have just eaten. A half-sandwich may be extended with a polite ¿Gustas? This can be confusing if you consider that the grammatical subject of gustar usually refers to the thing being liked, not the person doing the liking. Nevertheless, ¿Gustas? with no indirect object just means ¿Quieres? For example, ¿Gustas un pan? = Would you care for this pastry I’m offering you? Overheard in a cafe: ¿Gustan algo más? Would (the two of) you like anything else? A friend once rejected my offer of shared food but later accepted the drink by saying: Sí, gusto.
If a friend doesn’t offer you a bite, you may get a laugh by countering with No seas malito. ¡Móchate! = share it with me. Or if you want them to treat you to a whole one of your own: Móchate con un helado. A whiny voice adds a nice touch.
If your own offer to share is turned down, you can reply humorously with Qué bueno, más para mí.