Hmm ... interesting ... so if water leaks into the portafilter then you need a new group gasket to make a proper seal and no more leak?
The one you link to ... that will fit all standard E61 groups?
Water leaking INTO the portafilter? Usually we're concerned about leaks OUT of the PF.
These gaskets will fit an E61 but I use them for Gaggia home machines: 8.5mm
For E61's I recommend the 8mm gasket.