No doubt it is not up to the P-1 or-8 capabilities, or the P-3C either.
I'm actually far less sure about that.
The thing about MPAs (and any other sort of large fixed wing ISR or AEW&C aircraft) is that their capability is significantly dependent on the quality of their sensors and datalinks (and weapons in the case of MPAs). Second most important is the aircraft's range/endurance, as well as the weight of sensors and weapons it can carry -- and all of these factors in turn relate to the MTOW of the aircraft.
In terms of MPAs, important sensors include:
-surface search radar
-EO/IR ball
-sonobuoys
-MAD
-hydrocarbon sensor (for P-8)
-and all the associated datalinks and processing onboard to integrate and share all of this information
Weapons obviously include air dropped torpedos generally, but also AShMs, depth charges
How good are Y-8GX6's sensors in those regards? Well, it's hard to say, but the initial P-3C was produced
1969. I somehow doubt that Y-8GX6's sensors are inferior to those first developed nearly
half a century ago. If you're talking about upgraded P-3Cs, then that's a different matter but there have been so many different upgrade packages for P-3Cs that have been done, it's really hard to pin down how "capable" each of those individual iterations really are.
It's also important to consider that the Y-8GX6 should be seen as the culmination of many preceding aircraft projects and technologies, including the original abortive SH-5,s to the initial Y-8X MPAs, as well as subsequent AEW&C and ELINT/SIGINT aircraft, all of which would have been very important to allow them to develop more modern and competitive subsystems like surface search radar, EO/IR ball, and datalinks, to apply on an MPA. I would not be surprised if things like MAD and sonobuoys had also seen multiple iterative generations before settling on a type with performance that the Navy is more satisfied with.
As for the Y-8GX6's payload, endurance/range etc... that is something else we do not know, but we do know that the aircraft it is based on (Y-9) is said to have an MTOW of 77 tons, which is significantly higher than the P-3C or the original Lockheed L-188 it is based on. Of course, one difference between the Y-9 and the P-3C as well as the P-8 and even P-1, is that the Y-9 is a (significantly modified) variant of a tactical transporter rather than an airliner or even a fresh design like the P-1...
So the closest analogue to the Y-8GX6 is probably the SC-130J Sea Hercules proposal by Lockheed, which is a rollon/rolloff variant for C-130Js to give them MPA/ASW capabilities... And Lockheed has promised a 1325 nmi (2454km) combat radius with 4 hours time on station, which I imagine is not dissimilar to what the Y-8GX6 could offer. Of course, the SC-130J's number is likely with external fuel tanks (as do all C-130 variants), but at the same time the Y-9 which Y-8GX6 is based on is a larger aircraft than C-130J as well as even the C-130J-30.
(It is also interesting to note that P-8A has a combat radius of over 2200km with an endurance of four hours as well... though of course P-8A operates at a different flight profile/altitude than turboprop MPAs.)
So I definitely would not be willing to assume that Y-8GX6 is significantly non-competitive at all just because it is the Navy's first true ASW MPA, considering all the precursors projects and industrial capability that they are currently at to inform the development of key subsystems, not to mention they are using a very capable capable platform (Y-9) to develop their ASW MPA from in the first place.
I suspect part of the reason people are willing to be more dismissive of Y-8GX6 is because the P-8 (and possibly P-1) are the new and shiny toys with turbofans that make older P-3 variants and turboprop ASW MPAs look "old," so the Y-8GX6 being turboprop automatically relegates it into the "old" category. That, combined with the idea that Y-8GX6 is the Navy's first true ASW MPA, probably creates the sense of it probably being a deficient or limited platform.
This isn't to say I think the Y-8GX6 is necessarily as capable as P-8A or that its subsystems are going to operate without hiccups initially -- and I'm quite aware Naval Aviation will take quite a few years to get enough aircraft in service and experienced flight crews and operators to be able to take full advantage of their aircraft's capabilities. But I do think the gap between Y-8GX6 and other ASW MPAs may be significantly smaller than what has been suggested, especially in terms of the advancement of onboard subsystems and the range/endurance of the aircraft.