I checked ArcGIS Online Health Dashboard and nothing seems out of the ordinary, but they did do updates late night. I'm assuming this gallery is ArcGIS Online based?
The ArcGIS.com system will be undergoing maintenance on September 20th, 2017 between the times of 5pm and midnight PST. While we don't anticipate any significant downtime, you may experience some minimal disruption during this maintenance period.
FWIW - in Firefox I'm just getting the header and a blank popup with an "OK" button. Using the developer tools, after hitting the OK, I'm getting an error about a font not being available (404 error)
http://maps.cityoftulsa.org/MapGallery/themes/styles/font/fontello.woff?5490465
and a 503 - service unavailable error.
http://cityoftulsa.maps.arcgis.com//sharing/rest/community/groups?q=0709b291a39e4201abd647ca124c7b67...
In case that helps any.
Tagging ArcGIS Online but you may want to contact tech support too.