gs.daysAgo() returns a GlideDateTime object.
sys_created_on returns a string.
You are comparing apples and oranges.

All values are stored in the database as UTC.
They are displayed in the Users local time zone of system default.

I'm not going to provide you with the correct API here, because I don't think this is the best solution for this problem.

Use Flow Designer. 

Trigger on insert of asmt_assessment_instance.

Enter loop.

Wait 7 days.

If asmt_assessment_instance is still not completed, continue in loop.