Generator wiring into Automatic Transfer Switch – wire gauge
You have to run 4-wire to a generator.  A separate neutral and ground wire is mandatory.  A ground rod will not substitute. The only way to avoid running a 4th wire is to use non-flex metal conduit the whole way.  (or run 120V only, but that's no fun).
But that's OK, I'm about to save you enough money to pay for the ground wire or the conduit.  Minimum ground wire size is #8 copper or insulated #6 aluminum.
Since the generator run will be circuit breaker protected at 100A, you can use 100A rated wire.  Voltage drop will not be a concern at only 80 feet.
That is #1 aluminum wire if a) the wire or cable insulation is a type permitted 75C operating temperature, and b) the terminations (lugs) are rated for 75C operations.  The breaker panel lugs certainly are; whether Generac graces you with 75C lugs is a question for Generac.  you would think...
Do not put cable in conduit if you can possibly avoid it.  It's not illegal; just exceedingly difficult and serves no useful purpose.
If your entire run is in conduit, it behooves you to use THWN-2 or XHHW-2 wires which are indeed allowed 75C in practical use.  (technically they could go 90C but that requires meeting a long list of conditions; the electrical inspector is likely to nix this).   NM is not allowed outdoors and UF is allowed only 60C.
If your entire run is not in conduit, and making it so is impracticable, you can transition from one cable type to another in a reasonably large box using splices. The box must remain accessible without tools or dismantling any part of the house.