Multics MR12.9 is under active development. The current release is MR12.8, released August 8, 2023: https://multics-wiki.swenson.org/. While Multics no longer commercially supported, many of the current developers were previously involved with the project from its time at MIT and Honeywell Bull.
While we do want to support GCOS, it'll be GCOS-3, and that is if we can find system tapes. We are actually planning another blog post soon that will touch on this.
Now, Multics does have GTSS (GCOS Time Sharing Simulator), which Dean has helped us get working.
This facility is based on GCOS-3 4JS3, and it does work, although the software that we have available for it is minimal.
GCOS-8 is a different beast entirely. While there is the possibility that a future version of the simulator could theoretically run GCOS-8 in the future (as most of the work necessary overlaps with supporting CP-6), this is not something that we're going to support as GCOS-8 is a current commercial product.
I'll give an unpaid plug to https://www.softintegration.com/ for their Ch product, which is beyond excellent (and has existed for a very long time.) Clang-Repl/Cling is, however, nice to see, especially as it is open source.
It should be noted that not all of the hardware is strictly "emulated" (or simulated, if you prefer that term) in the traditional sense of interpreting each hardware instruction. The DPS in the name means "distributed processing system", and this is not merely a marketing term.
The DPS series, like many other mainframes, have multiple independent systems working together. For example, IO is handled by a FNP (frontend network processor) which was a (physically) separate minicomputer, with its own CPU, OS, etc. There were various FNP models (16-bit and 18-bit systems) produced. The DPS8M software implements the FNP at a high level, rather than running its original software (though there is both an FNP software simulator and a FPGA FNP project underway).
We did have some internal debate on if the system is a "simulator" or an "emulator", but really, it can be both, depending on how pedantic you want to be.
The biggest reason for the name being what it is, however, is that since the beginning, the software was called the "DPS8M Simulator", and even if emulator might be (arguably) a better name, it's not worth changing it at this point.