Hi Syave,
Filter is a JQL query of the project this is not same for all .If there is no filter you can use the Project name directly and select Statistic Type as Assignee/Repoter (as per the requirement) on the gadget this will show all the assignee workload .
If you still want filter then go to Project --> Issues-->Basic -->Select the project name and similaly other option as per the requirement and save the filter.
Use that filter in the gadget .
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