Hello,
I hope you are impressed by these results. That score, as is, it is really high and although it might be taken for granted, we did put a lot of effort to make Gantry score that high 
Now, regarding some suggestions to improve those C results, I can tell that gtmetrix is being a bit too severe on those results, but fortunately something you can easily adjust.
You can disable the query strings from the Gantry system plugin, it is an option that we added to ensure that assets get refreshed on update and is one of those false-positive results. Even though gtmetrix says it's bad practise, the honest truth is browser know how to cache assets with query string very well and don't affect page performance at all.
Go to Manage -> Plugins -> Filter for 'Gantry' and pick the 'System - Gantry 5' -> Disable Asset Timestamp
Regarding the charset, that unfortunately is not related to Gantry but to your server setup.
One easy attempt you can try is to add at the end of your .htaccess the following:
AddDefaultCharset UTF-8
AddType application/x-font-ttf .ttf
AddType application/x-font-woff .woff
AddType application/x-font-woff2 .woff2
AddType application/vnd.ms-fontobject .eot
That alone should be set the charsets the way PageSpeed is suggesting to do.
Hope it helps!