Sometimes people equate a really strong serving of espresso with a really bitter serving. It is possible the Gaggia is making a better brew of the beans you have. If you switched brands of beans at the same time as the machine you will have no baseline measure to work from. To know if it is bitterness that you became used, rather than strength, to you could try to take a longer shot from the Gaggia, or take a second shot from the same beans and see if the taste is something familiar to you. Usually an espresso double shot is just 2ozs, but often machine are planned for use making lattes and such, and the shots pumped are greater than 2ozs, sometimes as much as 4ozs. The bigger the shot, the more bitter. I don't know if this will help you, but most of the Gaggias are very good machines, so it is worth finding the pathway to what it is you like.