The exact GPUs in the new iMacs are in my chart below. All of the info can be found on Nvidia's product pages and other sources such as notebookcheck.net
Please note that the MX versions are the faster versions of the M mobile GPUs. The 680MX in particular is brand new and only appeared on the Nvidia website this morning. It definitely was not there yesterday.

Please note that the MX versions are the faster versions of the M mobile GPUs. The 680MX in particular is brand new and only appeared on the Nvidia website this morning. It definitely was not there yesterday.