Documentation
Can you look up how to use it?
We couldn’t find an official manual for Grsync, and there seems to be no official forum. Fortunately, the program is well known online, and web searches provided some tutorial material and other information from various sources. In its favour, rsync itself is well documented, and you might be able to figure out how to do something slightly out of the ordinary by looking up how to do it with rsync.
BackupPC has an online manual that covers every part of its huge feature set. There’s also a Wiki with some how-tos on the website. There was an official forum, but it’s now closed down and in read-only mode. There is, however, an active email list.
Back In Time has an online manual. It’s fairly brief, but it isn’t a hugely complicated piece of software, and the manual covers every part of the user interface. There is a detailed man page ( man backintime ) that covers the fairly extensive command line facilities of the program.
Vorta also has an online manual that covers the whole program. Beyond that, the underlying technology behind it, BorgBackup, is well documented, and you might be able to find out how to implement an unusual requirement through that. For example, you might need to look into BorgBackup’s documentation to figure out the syntax for Vorta’s file and folder exclusions.
The Deja Dup website is underwhelming when it comes to documentation, but it’s difficult to argue that such a simple program requires extensive documentation. There are plentiful YouTube tutorials to assist the beginner, too.