When you unscrew the group valve (this spring-loaded pea valve that Colin is referring to), you may find that
i) it has scale accumluations on it (as might the shaft that it seats within);
but you might also find that
ii) the plastic pea has deformed from heating cycles and just normal wear and tear. This can also prevent the valve from sealing properly, and is a simple replacement available from vendors like
. Expected cost around $8-10, iirc.
If you order a new group valve, tack on replacement parts that you will eventually need: group head gasket(s), shower screen(s), maybe a spare basket, and (if they happen to have them cheaply, dunno if they do) thermal cutoffs (thermal fuses) to match your machine ($3 part).
I am assuming that this is a new model Gaggia Coffee when you say that you have had it for a year. Some of the parts on the much older original model Gaggia Coffee (white steel exterior with a front-mounted steam knob) were very different.