Did you download the very latest x3_installer.php? We update this script often.
https://www.photo.gallery/downloads/
OFTEN, failure to download via CURL would be if the download link in the script includes a REDIRECT. For example, if you are using an older installer that tries to download from imagevuex.com, which then redirects to photo.gallery, your server CURL will block this because you probably have NOFOLLOW for curl redirects (for security reasons).
If you are using latest installer and it still doesn't work, I will need FTP login to diagnose the problem. I can almost say for sure though, if CURL simply fails, this is almost certainly because your server has BLOCKED curl requests to external resources (eg resources on remote hosts). In which case, there is no way you can use any "installer" script on the server. In the X3 installer we 1) test for ZIP extension, 2) check for CURL extension and 3) make a test CURL call to a remote file test.zip on our server. On your server 1 and 2 succeeds, while 3 fails, in which case your server just does't want to load external resources.
It could be our firewall (cloudflare) considers the request from your server IP a "threat", and therefore blocks it, but I find that highly unlikely.