It is a lot easier to diagnose a potential problem if we can see the process you go through each time you make a coffee.
Ive never used a Gaggia so I cant comment specifically on that but instead of just thinking temperature of the water, think about the time that the coffee is in contact with the water.
For example if you grind your coffee, tamp, and lock the group head in, and then leave it in while youre texturing your milk, the coffee is sitting in the group heating up. If your shot runs very slowly and youre getting over extraction this could be another problem which you have mentioned.
Of course you may already know this, which is why if we see a video of your general routine, we might be able to pick out some problems which may be causing the bitterness.