Probably guy vs. gang, unless they are attacking just one guy and only that one guy is fighting back.
Like, if Cypress (PC) runs into The Jackals and they unload on each other, full auto, that's guy vs gang. If Cypress specifically targets only Richmond, it's still guy vs. gang if all the Jackals are firing back (but you should apply the consequences of Cypress' harm on the gang to Richmond first, because that follows from the fiction and is you being a fan). If they're not all fighting back, Cypress is probably going aggro, unless for some reason the Jackals are letting Richmond take on Cypress alone. Maybe this is the case, while the rest of the gang fights Chewy (another PC). If so, Cypress vs. Richmond is guy vs. guy, and Chewy vs. the Jackals is guy vs. gang.
If Cypress and Chewy wade right into the ranks of the Jackals waving machetes, have them both roll seize by force, guy vs. gang. However! If the gang is a medium-sized gang, and Cypress and Chewy have a good attack plan that splits the gang in two, have them each fight against a small gang, as a reward for good tactics.