If torquing the portafilter tighter doesn't seal, then the gasket is probably dry / worn out. Removing it without damage is possible, but it isn't easy. Let's begin with a short review: E61's are pretty much all the same -- the gasket is wedged in a groove and the gasket holds in the dispersion screen.
The article E61 Group Maintenance
has lots of pictures and instructions on how to clean and replace the grouphead gasket / dispersion screen. It suggests using an awl / pick to grab at the gasket, but I've squeaked it out with no damage two different ways:
- If you're lucky, the dispersion screen has a groove on the side that you can catch with a blunt screwdriver. Pry just a little every few inches around the perimeter, using the group for leverage (that's why I prefer the screen below on the right, it's easy to remove).
- Otherwise, try working a dull, thin, flexible knife blade (I use an old el-cheapo "Papered Chef" paring knife) along the inner edge of the basket, pulling the gasket downward and outward a little. Go around the perimeter; if you're patient, it will come out without damage in less than two minutes.
If the gasket is shot, then there's no need to worry about nicking it. Follow the article's instructions and reinsert a new gasket / screen.
PS: There are two types of dispersion screens for the E61. The "ungroovy" ones (left) usually have a thin metal ring spot-welded around the perimeter of the screen and a "dot" welded to the center, whose purpose is still a mystery to me. I've gotten both kinds off without nicking the gasket, but the ungroovy ones are more challenging (and I scratch the sides of them more
PSS: Also see What is that ugly Allen screw for?