Output when compute GC content is different between using user input and using string
Is it cheating to use Biopython(?), will a solution might be helpful anyway. This assumes you have your sequences stored in the seq.fa FASTA file:

from Bio import SeqIO
from Bio.SeqUtils import GC

print(max(((r.id, GC(r.seq)) for r in SeqIO.parse("seq.fa", "fasta")),
          key=lambda x: x[1]))


Output:

('Rosalind_0808', 60.91954022988506)