is there any way to keep the grades of participants shown in the grade book after unenrol them?
If you suspend the participants rather than unenrolling them, their grades will still be available in the gradebook. Please see the documentation Unenrolment for more details.
I know that and I'm doing like that after the trainees attend their courses I suspend them from the course, but like that will be a hug records show in the grade book, so I'm looking for any other way to can show the grades of unenrolled trainees from the main Gradebook of the site directly.
You can set your gradebook to only show active enrollments - this is done in your grade report settings. So the grades are still there but not visible.
i know that i'm asking about the grades for the unenroled trainees, if there is any plugins can do so!