The option exists to trade Jimmy G closer to the preseason or even during the season, when you've had a chance to see this year's draftees and a chance to feel out how Brissett is doing and whether he's ready for primary backup duty.
There is no rush here. In fact unless someone really steps up to the plate with a killer offer I don't expect the Pats to trade Jimmy G before the draft, or even before the preseason is over. Too many factors that add unnecessary risk to the equation and time is not a factor. Let some time evaluate its options at QB in the preseason, go "oh holy *****ing **** this will never do!" and make a desperation deal.