Gday Michael and welcome to CS.
I dont know how handy you are when it comes to pulling things apart, but if you are, there are some hints on this link as to how to make the grind finer on your grinder (Solis 166/De-Longhi KG 100). Bearing in mind that this mod will no doubt shorten the lifespan on the grinder. If youre not mechanically inclined, you might have to get yourself a better grinder. The De-Longhi/Solis are good for machines with a pressurised portafilter, ie your old machine, but they are not up to the task for machines that dont have these devices, such as the Gaggia.
Good luck with it.