If I develop a parametric 3D mesh creation tool and license it under GPL (because it is dependent on some geometry libraries also under GPL), can I sell 3D mesh data for use with the tool and would these be subject to the GPL? Would the generated custom meshes be subject to the GPL too for the customers that bought these data and created/generated meshes from it using the tool?
In most cases, data that you read and write with a software tool (including GPL-licensed software) is not impacted by the license of the tool. For instance, if you edit your own document with LibreOffice it is not subject to the licensing of LibreOffice. So here, assuming that the mesh data you sell are yours, the custom generated meshes from that data would be subject to whatever term you please.