Upvote:3
Likely not. Different countries operate with slightly different guidelines and documentation requirements for determining dependency, but the UK authorities guidelines state that your grandfather must be 'wholly or mainly financially dependent on the EEA principal (you) to meet his or her essential needs in order to qualify'.
Your obligation letter only establishes a dependency for the costs directly related to your grandfathers visit in Germany and not any permanent dependency, as rather required for a family permit.