Does a.gtld-servers.net have a list of all .com domains?

Yes, the "x.gtld-servers.net" are the authoritative servers for the "com" top level domain, so they have all the "pointers" for the .com domains. You can see the nameservers for the TLD by running

dig -t ns com
dig -t ns us
dig -t ns dk
dig -t ns aero

Do a query for the domain itself – dig @a.gtld-servers.net com. – and look for the "authoritative answer" flag:

snowflake ~ $ dig @a.gtld-servers.net com | grep flags
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
             ^^