We get close to 20,000 emails daily from either gmail.com or Google Apps domains and haven't been experiencing any problems whatsoever receiving these emails in SM 16.
Do you have your SMTP Logs set to "Detailed"? If so then SmarterMail should log the exact reason why those emails are being rejected/bounced.
Without seeing the detailed SMTP Logs you'd want to make sure that you do not have a wildcard under SETTINGS > SECURITY > SMTP BLOCKS that may be inadvertently blocking either gmail.com accounts or Google's EHLO (Google's Mail Servers follow the EHLO convention of mail-ab#-c##.google.com) or alternately that you do not have a Custom Rule set under SETTINGS > ANTISPAM > SPAM CHECKS that is enabled for SMTP Blocking that they may be triggering (easy to accidentally do with custom RegEx rules).
If there are absolutely no entries in your detailed SMTP Logs for the search string of "gmail.com" then they are blocked by IP under SETTINGS > SECURITY > BLACKLIST. Last time I checked Google uses the following IP Ranges for Gmail/Google Apps:
You'd want to make sure that you don't have any Blacklist ranges that these may fall within.
Other than that, Antispam settings perhaps being set too strictly for SMTP Blocking or using an RBL for SMTP Blocking that is returning a negative (failure) response (common if you aren't using your own DNS Servers, exceed your hourly/daily limit for free RBLs...most have a max of 30,000/day, or are using an RBL that requires registration of your DNS Servers or Mail Server's IP).
Your Detailed SMTP Logs will know for sure though (i.e. "554 Sending address not accepted due to spam filter" or "Mail rejected due to", etc). If you can post of a sample of a rejected STMP connection we can help you find precisely where in your SmarterMail settings to alleviate the problem.