This is an attempt at explaining the events of this timeline before the main fic happens.
The events of Sonic Forces take place in September 22XX, over the first 2 weeks. This is the reference point that we're starting with, and all other dates will be measured in relation to this.
Zero (known as the Ultimate Mercenary) is an active mercenary and soldier. He usually does missions with his squad, known interchangeably as the Jackal Squad or Squad Jackal, but he sometimes does solo missions.
The Jackal Squad's main motivation for committing crime is to have enough money to support themselves; as Jackals are rare in the Sonic world (and the ones that are alive are widely considered bad omens), they have no supporting structure and must fend for themselves.
Their main modus operandi is breaking into museums and stealing artifacts, holding them for ransom, which the museum curators almost always pay. Their second MO is to break into people's homes, steal their stuff, and sell the spoils at less-than-reputable pawn shops for cold hard rings. Between these two crimes, they usually have enough money to feed themselves. If they ran short of cash, they would take paid jobs from rich people.
Meanwhile, Gadget is 17. He lives with his mom, his dad, and his younger brother, Gizmo. They live in the Hill Top Zone, of Sonic 2 fame. Most of the Mobians living there are rich, but that just makes them obvious targets for robberies.
Early that month, Zero and his Jackal Squad go on a robbery spree in the Hill Top Zone. They make away with almost 60,000 rings worth of items. During the spree, Zero and Gadget meet for the first time, when Gadget hits him over the head with a broom for trying to steal a family heirloom. Zero wins the fight, obviously, since he has a sword, but he decides to leave the heirloom and leave Gadget alive.
Later that month, the Mystic Jungle incident happens. The Jackal Squad, for some extra rings, are defending Eggman's base in the Mystic Jungle. Shadow comes in and attacks the base, killing the entire Jackal Squad, save for Zero.
Zero has the Phantom Ruby implanted in his chest, and he becomes Infinite. The events of Project Shadow occur.
The events of Sonic Forces-proper begin, as Infinite defeats Sonic and captures him for Eggman. Sonic will be tortured for the next 6 months, as in canon.
Sometime between the defeat of Sonic and the events that happen below, Gadget is on a recon mission snooping around an Eggman base when he finds a shiny red rock that emanates power. Unbeknownst to him, he's found a Phantom Ruby prototype. (This replaces the canon scene where Gadget finds a Phantom Ruby prototype, as in this AU, Gadget had the Phantom Ruby prototype since basically the beginning.)
On May 10th, 22XX, Gadget turns 18.
Eggman attacks a seaside town. The Resistance mobilizes quickly to push the Doctor back, but their forces are thrown into disarray when Infinite shows up to save the Doctor's behind. Gadget gets in some close scrapes, and Infinite sees him. The thought of the wolf just won't leave Infinite's mind, and it doesn't help matters when a dazed and confused Gadget stumbles into his living area. Infinite bandages Gadget, and the two quickly fall in love with each other and make out on Infinite's couch.
The next morning, Infinite promises Gadget that he'll be spared when the Empire wins. Gadget, in turn, promises Infinite he'll do everything in his power to ensure the same for him if the Resistance wins.
The events of the game wrap up in about a week real-time. Sonic and Gadget defeat Infinite, who ends up dying after the fight. Sonic and Gadget go on to defeat the Death Egg Robot and win the war. Then, the events of the fic happen.