gca.shは安全ですか？

I trust this website itself is good, since this appears to be nothing more but a redirection tool. HOWEVER, spammers and most of all scammers also discovered this service, and today I've been spammed by a spammer using this service multiple times, and I was not the only one. Therefore you should only use the links provided with this service when you received them from people you know and trust. If you don't know them, don't click the links as there's no telling on which (possibly malicious) site you can end up. (And basically this goes for all redirection services).