Tiled Mosaic Breaks Site

I’m afraid that error is quite common. You can learn how to fix it here:
http://www.wpbeginner.com/wp-tutorials/how-to-fix-fatal-error-maximum-execution-time-exceeded-in-wordpress/

However, given that your gallery includes so many pictures, another solution could be to split the gallery over multiple page, thanks to WordPress’ pagination options:
https://en.support.wordpress.com/nextpage/

I realize that’s less than ideal, but it would solve your issue, while making the gallery a bit easier for your readers to browse, especially if they access your site from a slow internet connection, or from a mobile device.

I hope this helps.