Fix for the internal gateway is not a part of the 7.7.6 release. We are doing thorough testing to verify there are not unintended consequences and will update when we have more info. This is a significant issue from our perspective, and we want to make sure we get it right.
In the meantime, the current version of the shrink tool automatically alters file cache size during shrinking. There have not been any changes in 7.7.6 that would prevent it from functioning.
The tool can’t connect to the DB if Ignition went through some form of dirty shutdown, so try doing a clean shutdown (via gcu or the stop script) and run the tool again. If there is no .lck file in the data folder, it should be able to connect.