For the sake of comparison of how top goalscorers in Portugal have done compared to how they do in top 5 leagues:
Jonas in Portugal - 132 games; 110 G+ 32 A.
Jonas in Spain + Italy - 233 games; 87 G + 37 A.
Haris Seferovic in Portugal - 120 games; 57 G + 19 A
Haris Seferovic in Germany + Spain - 128 games; 21 G + 16 A
Bas Dost in Portugal - 84 games; 76 G + 10 A
Bas Dost in Germany - 121 games; 48 G + 14 A
Mehdi Taremi in Portugal - 152 games; 82 G + 45 A
Mehdi Taremi in Italy - 14 games; 0 G + 1 A
Jackson Martinez in Portugal - 141 games; 77 G + 14 A
Jackson Martinez in Spain - 15 games; 2 G + 2 A
See a trend here?
The outlier to this are players like Falcao, but Falcao in his prime was one of the best strikers in the world. Will Gyokeres be an outlier too? Are you willing to pay the 75-80m to find out?
That's the question Barcelona or any top club that need a striker need to ask themselves.