I find them really interesting, being a weak, communist parallel to the Foundation, and I was wondering what is written about the GRU-P after the collapse of the Soviet Union?
This is entirely up to headcanon. You can find two "official" reports for GRU-P in the GoI formats page.
It is a fairly small GoI at this point, and that is why it is up entirely to the reader to shape it. We do know that they were actually a decently strong military and scientific force, but after the USSR collapsed, they really disbanded and the GoI fell harder than IJAMEA.
This is what is said on the Groups of Interest page:
Originally known as the ЧД АКН ("ChD AKN", or Fourth Department Abnormal Occurrences Commission), it was established in early 1935 by direct decree of J. V. Stalin, its first task being the investigation of the murder of S. M. Kirov.
During WWII, the Commission expanded into the newly reorganized GRU as Division "P" - Psychotronics, working to counteract anomalous threats from the SS, Ahnenerbe, and the Vatican, and to capture and study anomalous artifacts both at home and abroad for the benefit of the Soviet government. Division "P" has carried on with this role throughout the Cold War, making Foundation operations in the countries of the Comintern difficult.
After 1991, rapid restructuring, coupled with budget cuts and the decommissioning or sale of significant assets, resulted in numerous defections of their personnel (A significant proportion of both Chaos Insurgency and MC&D operatives in Eastern Europe originated from GRU Division "P" ranks) and the flooding of the European black market with anomalous items formerly in their possession. Currently, this Group of Interest poses little direct threat to the Foundation.
If you're looking to write a story when they're in their prime, I'd suggest doing it during the Coldest War. But, if you're looking to write a story after their fall, do it any time following the USSR's collapse.
Pretty limited info, but here's some rambling:
This article mentions a modern FSB Division P. IMO this doesn't make sense as a successor: the actual FSB is the successor service to the civilian internal security portion of the KGB. The actual GRU is military foreign intelligence and, although they were heavily depleted after the Soviet Union fell, they never went away.
Another interesting thing that file brings up is civilian/Foundation negotiated paraweapon disarmament, along the same lines as nuclear disarmament. (I could've sworn there was another tale series along those lines, but this was years and years ago so it might be gone.) Anyhow, point being, GRU-P's main activity seemed to be acquiring and weaponizing anomalies. Given that the Soviets were willing to negotiate down their nuclear arsenal late in the cold war, it's not unreasonable to think they'd do the same with their paraweapons, meaning decreasing support for GRU-P's research up to and after the fall.
GRU-P previously tried to contain the Sarkic criminal organization The Hunter's Black Lodge, so some post-Soviet Division P remnants may either still be fighting them or have defected to them.
Beyond that, the GOC calls GRU-P Scarlet Dagger. There's another group they call Scarlet Hammer is ex-KGB elements who traffic paranormal arms as part l of the "Viktor" mafia. These are barely mentioned, here and here, so there's not a huge canon background there. The idea that some GRU-P went into organized crime is plausible.
Edit:
Oh, another perspective from the Third Law Hub
GRU Division "P"
One of the Soviet Union's two major paranormal agencies, GRU-P was created during the Second World War to protect the Soviet Union from paranormal and occult threats, and to utilize esoteric weaponry against the country's enemies. With the onset of the Cold War and the paratech arms race, the division's role was expanded to include recovery and reverse engineering of anomalous objects and paratech. In this role, it performed corporate espionage and theft of intellectual property on a number of companies, including Prometheus Labs.With the end of the Cold War, GRU-P has found itself as the sole remaining paranormal agency of the Russian Federation. Its operations are now focused largely on dealing with the black market for anomalous objects and paratech that has arisen in post-communist Russia.