You can use a High Density 8 port FXS module. This will need to be plugged in to the existing GW and you can connect analog phones to it.
Some info on it is given here:
http://www.cisco.com/en/US/prod/collateral/modules/ps3115/product_data_sheet0900aecd8016c1c6_ps5855_Products_Data_Sheet.html
You can also use a couple of VG204s (not sure but I think they are End of Sale)
There is always the option of a couple of VWIC-4FXS cards.
I am not sure if these modules will work for you since they are not gateways in the true sense of the word (for that matter, even the ATA is not a GW).
HTH.