It's P-8 not P8 or P.8 as per the US 1962 Tri-Service designation system. Interestingly the Lockheed Neptune Maritime Patrol aircraft that the RAAF used before the P-3 Orion could be had with twin machine guns in the nose turret, although the RAAF didn't fit these. They did have bombs, torpedoes, mines and depth charges; and Eight 12.7 cm (5 in) rockets under the wings. Versatile and deadly!
I assume it was a joke, but you will never see a GAU-8 fit on the P-8. I am not sure where it would even fit.
The P-3 could also carry a large variety of weapons including unguided bombs and rockets, torpedoes, depth charges, mines, but importantly also later gained the Harpoon and SLAM cruise missiles and the short range Maverick missile. No guns though.
Guns, bombs and rockets were made obsolete for the anti-submarine role by both the advent of nuclear submarines which don't need to surface and the development of missiles like the AGM-84 Harpoon, which can be used against ships and surfaced submarines from much greater distances.
The P-2 Neptune and P-3 Orions were fine aircraft for the time, but were also developed in the era when anti-submarine and maritime patrol missions were still flown at low level, often using Magnetic Anomaly Detection (MAD), hence the piston and turboprop engines respectively, which are efficient at low altitudes.
Today sub hunting has changed and can be done from higher altitudes. There is a much higher reliance on very sensitive active and passive sonobouys to detect submarines by sound and the P-8 does not have a MAD capability as standard. The problem with flying at low level is that the horizon/curvature of the earth obscures the reach of radar and optical sensors and as such the P-8 is designed to operate at medium altitude and use its long range sensors and standoff weapons.
The P-8 today can already drop torpedoes and fire Harpoon like the P-3 and will be gaining JDAM (GPS guided bomb) and QuickStrike mine laying capability and it is to receive the capability to fire the AGM-158C LRASM...this is a big deal as LRASM is a long-range stealth anti-shipping cruise missile that the RAAF is procuring and looking to build locally for the F-18F Super Hornets. If the RAAF P-8s gain this missile it gives the RAAF a very long range anti-shipping punch.
Furthermore the AGM-158C LRASM was developed from the land-attack AGM-158B JASSM-ER which the RAAF is also acquiring for the Super Hornet. If this is integrated into the P-8 it effectively turns the P-8 into a long range strategic bomber.