Centred and resized gallery block code

Site URL: https://brianrolfekids.co.uk

I'm trying to replicate how this site behaves as it's a very similar design to my own and the gallery block resizes and centres regardless of how you resize the browser. I've found a code that keeps it centred vertically but it doesn't resize the gallery. Is there a code I can use to replicate the way this gallery behaves? 

Site example https://jessrubyjames.com

I think it may be on the Avenue 7.0 template but I'm on 7.1