hard to say from distance as it entirely depends on your setup, especially domain provider, dns-server etc.
sadly we can’t cover this here in terms of support, as this is most likely not hestia related and we expect you to be able to handle basic management/administration task on your own - like the way you would need to without hestia 
however, before dealing with gmail, how about you setup your domain and a single mail account on your hestia server first. then use a local mail client or even the webmail to check if mail is working at all.
if not check the via one of the bigger online page that help with testing mail setups (mxtoolbox f.i.) …
only after you have your inbox running on your server start connecting it to gmail - I guess that way would make it more easy, esp. for troubleshooting