Physicalists have several responses: 16+% of all philosophers actualy disagree with P1. See page 16: https://philpapers.org/archive/BOUWDP The reason would be a belief in the logical necessity of Identity Theory -- and that anyone who held that Identity Theory was untrue was simply confused.
36+% of philosophers reject P2. Our imagination is not rational and based on our ability to imagine irrational things, one cannot infer possibility from imaginability.
Accepting P2 does not necessarily leave an explanatory gap. There are several standard physicalist responses:
a) Knowledge shortfall -- consciousness is identical to either neurology or algorithms of some kind, but we don't know what kind yet, or how this happens. The "explanatory gap" is just a shortfall of current knoweldge, and at some point we will be able to close that gap, and explain how consiousness is Identical to brain or processing states.
b) Deny that we really are conscious, or have qualia. This is the strategy of both Dennett delusionism, and Churchland eliminative reductionism. If humans are zombies, then the "zombie argument" loses all its sting.
Whether any of these arguments are effective or not is something which cannot be addressed in a single reply on a discusison board, and requires reading deep into the Philosophy of Mind literature. I simply outlined the primary physicalist responses above, not whether they work.
Two sources I can recommend to start this process would be: Susan Blackmore's "Consciousness A Very Short Introduction", and Jaegwon Kim's "Physicalism or Something Near Enough". Both are, or were, physicalists, but are highly skeptical of physicalist arguments.
Blackmore focusses on psych lab experiments that refute the neural and algorithmic Identity Theory arguments, and also on the consistency of our knowledge of consciousness. She considers all physicalist models refuted, but physicalism to be proven by physics, so she ends up rejecting that consciousness even exists.
Kim depends on philosophical consensus, plus logic -- primarily related to causation. He too assumes physics has proven causal closure of matter. He holds that the consensus of the last 50 years of Philosophy of Mind is that qualia exist. He shows using causal logic that Supervenience must either reduce to neural identity theory or break his Causal Closure assumption (and he thinks that algorithmic identity theory is a dualist worldview, despite the protests of the algorithmecists). And he cites the consensus of Philosophy of Mind that experiential qualia cannot be reduced to neurons. This leads him to an epiphenomenal dualism for portions of consciousness.
These two authors are in conflict with each other, but that is where Philosophy of Mind is today. There is no consensus view.