aaPanel_Captain This is correct to one point, if the deployed mail server on aapanel is missconfigured, one will get all mails marked as SPAM, I had opened several threads about missing features in DNS side and on MAIL side, that connects to the two, a mail server without proper SPF,DKIM and DMARC in place will end up always on spam. AAPanel fails on educating its users about this importance, thinking all should know, I realised based on posts here 60 % or more of the aapanel users are newbie this concept and lack overall knowledge that a sysadmin would have or a devops. Thus resulting in these ... unresolvable threads that because of lack of knnowledge dod not provide all details, so I would suggest to make own tests on aapanel with the above mentioned options and make sure you test mails yourselves.
There is not such thing as a GMAIL whitelist when it comes to a mail server.
Thus would require you to have access to all GMAIL you are mailing, that wont happend, since the whitelisting you speak about is done at user/account level. They do not have same capabilities like Outlook/Exchange has, there one can whitelist his organization and control whitelists blacklists for a domain. Thus you wont be using AAPanel mail server, yet exchange.