How do i install Napoleonic wars from the talegames website? i downloaded the setup but it says i didnt install mount & blade warband?

I am very sorry but you will need base game and DLC from same vendor. Maybe talk to Taleworlds about changing the DLC to a GOG key IF you have the base game, Mount and Blade Warband, here on GOG.

If that does not work, you might be able to get a copy of Warband on their website too if you prove you own it here on GOG already.