It does sound as though water may be getting under the seal. It should not be able to get behind the seal while the portafilter is locked in tight, but maybe it could be happening as the portafiler is removed.
If there is any damage to the flange in the group head that the inner edge of the seal fits against, or if the inside diameter of the seal is too big and it doesn't fit firmly against the flange, it could let water in.
However, I think that there would have to be at least some pressure in the group as the portafilter was being removed, to force water under the seal. Do you hear any release of pressure as you turn the handle to remove it ??. If you do, then a faulty solenoid or valve may well be the cause of your problem, but if not, I'm not sure what else it would be.
There is a forum at Gaggia Users Group who are very experienced and helpful with all the small Gaggia models, you could try posting your query there.