Well indeed, it would seem to be an extra addon to Imperia.
I'm just downloading it now ( the file on the left side of that page who is rougly 84MB), though it goes epicly slow, so 'ill see exactly what it is when it has finished downloading. I remember when i downloaded imperia from this site it went epicly slow, so i stopped and got the german version which i got in less then 5 minutes.
As it says somewhere at the bottom of the page: you should install the mods03.gs1 into your imperia folder ( take a backup and try it on that, because there is already a mods03.Gs1 file in the original imperia. I dont know if its a whole new mods03 or an updated, so to be on the save side, copy and try from there.
As to what it exactly changes, i think if i read it correct, it adds new units for england for example, funny enough they took them from Divided Nation. Im pretty sure there is a copyrright issue here since DN was made by ROS and not gsc/cdv. Or they got a permission to do this.
I'll see when it finished downloading, ill try to install it, but that will be after diner time.
Thanks for that link cichor, i completely forgot about this.