Whoa! If you going to read Gutmacher then read it all, taking him out of context will get you killed (yes I've read his books). He is big on pushing not pulling your gun as a hostile act and getting yourself charged with aggrevated assault with its three year mandatory. Gutmacher will scare you out of ever pulling your gun in fear of that 3 year sentence. (BTW you almost NEVER see it make it to court unless it was truly agressive, usually it is pled down to a misdemeanor, but I digress.)
However in the example you should be reading 776.031
776.031 Use of force in defense of others.--A person is justified in the use of force, except deadly force, against another when and to the extent that the person reasonably believes that such conduct is necessary to prevent or terminate the other's trespass on, or other tortious or criminal interference with, either real property other than a dwelling or personal property, lawfully in his or her possession or in the possession of another who is a member of his or her immediate family or household or of a person whose property he or she has a legal duty to protect. However, the person is justified in the use of deadly force only if he or she reasonably believes that such force is necessary to prevent the imminent commission of a forcible felony. A person does not have a duty to retreat if the person is in a place where he or she has a right to be.
You may use force which could be considered to include brandishing to stop the trespass, but in this case I beleive you would have reasonable reason to believe that a forcible felony either IS or IMMINENT in which case you could use deadly force.
I see a guy being beaten by a bunch of thugs as aggrevated battery and the woman being assaulted is by definition sexual battery BOTH ARE forcible felonies in Florida
776.08 Forcible felony.--"Forcible felony" means treason; murder; manslaughter; sexual battery; carjacking; home-invasion robbery; robbery; burglary; arson; kidnapping; aggravated assault; aggravated battery; aggravated stalking; aircraft piracy; unlawful throwing, placing, or discharging of a destructive device or bomb; and any other felony which involves the use or threat of physical force or violence against any individual.