Thomas Gage was the leading British military leader in the colonies before and during the American Revolution. His proclamation in June of 1775 was a final attempt, after the events of Lexington and Concord in April and the start of the war, to stop further conflict by offering a pardon to all those who laid down their weapons and protection to those loyal to the king.

Well, almost all—he makes an exception for John Hancock and Samuel Adams, "whose offences are of too flagitious a nature" to avoid punishment. (Source)

(By the way, "flagitious" means "shamefully wicked," so feel free to add that to your everyday vocab.)

You've seen this move in pretty much any movie with an evil villain attempting to take over the world: submit to me, and I'll spare you. Gage wasn't necessarily an evil villain, he was just a Loyalist with power, but you can see where that cliché probably came out of some real-life events.

Gage's proclamation is kind of the anti-Declaration of Independence, attempting to argue that the colonists who have been fighting against the British are the ones who have been unjust, inciting unnecessary violence against their benevolent sovereign. He says that the press has convinced people through unfair, inflammatory language that they should hate the British, leading to cruelty towards members of the British government.

Unlike Jefferson and the Founding Fathers, Gage doesn't go into a whole lot of detail about the colonists' offences, which are, "too many to enumerate on one side, and are all too attrocious [sic] to be palliated on the other."(Source)

There were still a number of Loyalists in the colonies, so Gage's words wouldn't have fallen completely on deaf ears. Compare his language in this official proclamation to Jefferson's in the Declaration, and you can decide which is more effective. Gage's language tends to be more tempered and distant, although also pretty accusatory towards the rebelling colonists.

He doesn't present any revolutionary ideals like Jefferson, which is maybe one reason why he didn't find swarms of colonial subjects laying down their guns at his request. Stern requests don't get quite the same response as lofty ideological statements, generally speaking.