How can I hide a group of tasks? Or maybe make a task that is completed be hidden?
Thank you.
No such feature, sorry.
Relevant feature requests in the tracker:
Hello! Using GP v3.3.3316, portable with .zip. JRE 17.0.17.
Seems that a tasks with finish date in the past (from today) and less that 100%… does NOT show in the filter (past tasks)… in my project.
BUT, in a new project, easy test case does work and filter shows past tasks…
Not able to reproduce the issue when filtering past tasks. 
Can you share the project?
Hmmm… Not directly… I will try to build an anomyzed version to send, as soon as I can.