Hi @ThimiosSp
Try the following:

You have the first step.
The List row is:

The FetchXML is
<fetch top="10">
<entity name="contact">
<attribute name="contactid" />
<attribute name="emailaddress1" />
<filter>
<condition attribute="emailaddress1" operator="eq" value="" />
</filter>
</entity>
</fetch>
Your then insert the From field.
The list returns the list of Contacts matching the From field. You'll need to decide what to do if you have duplicates, I've assumed you'd only update the first returned.
The condition is

The expression is: length(outputs('List_rows')?['body/value']). The Terminate stops the flow if there isn't a match.
The compose gets the first Contact Id

first(outputs('List_rows')?['body/value'])?['contactid']
The Update a row is then

I have a basic version of Dynamics/Dataverse and there isn't a GDPR field in that either. I suggest you add a new field and you'll be able to set it in the update flow step.
If this works for you, please could you mark is as the solution to your question.