Code Exemple:
gsutil cp \"http://www.exemple.com/file.txt\" \"gs://bucket/\"
You can stream the output of curl to the gsutil cp command as follows:
curl http://www.example.com/file.txt | gsutil cp - gs://bucket/file.txt
There is Cloud Storage Transfer Service with an option to upload list of URLs though not very simple and more batch-oriented.
For wget:
wget http://www.example.com/file.txt -O - | gsutil cp - gs://bucket/file.txt