Can only compare identically-labeled Series objects error with if statement - Python

I'm attempting to run an if statement to match the country of origin of marathon winners to theirs countrie's gdp data. I am getting the error 'Can only compare identically-labeled Series objects'.

if df['Winner Country'] ==  gdp_data['Country']:

    if df['YEAR'] == 1970 :

        df['gdp'] = gdp_data['1970 gdp/cap'] 

gdp_data example:

Country 1970 gdp/cap    
Kenya   98  

df example:

YEAR    Winner_Name Winner_Country  Time    Gender  
1977    Dan Cloeter USA             2:17:52 M   

I intend to assign a gdp value to df based off both country and year(I only included partial data, there are extra columns for each year in the gdp_data datarame).