For this Setup piece, we draw heavily from Google's own documentation.  Please start with this page:

https://support.google.com/a/answer/2685650?hl=en&ref_topic=2921034

This page is a refresher on mailflow and some terms.

In the GSuite portal

Inbound FROM Reflexion

https://support.google.com/a/answer/60730?hl=en&ref_topic=2921034

On this page, you will setup your Inbound Gateway.  This setting will allow mail that is filtered by us to be exempted from DMARC and DKIM scanning.  You can also tag messages that do not come from our IPs as spam, or flat-out block them. Please add the following IPs to the table:

69.84.129.224/27 (255.255.255.224)
208.70.208.0/24 (255.255.255.0)
208.70.209.0/24 (255.255.255.0)
208.70.210.0/24 (255.255.255.0)
208.70.211.0/24 (255.255.255.0)

Outbound TO Reflexion

https://support.google.com/a/answer/178333?hl=en&ref_topic=2921034

This page has the setup instructions to send to our Smarhost.  Once you reached the Outbound Gateway field, please enter our smarthost address and hit save:  asp-submit.reflexion.net

We strongly recommend using the FQDN for our smarthost rather than the IP.  We do not expect to change our IPs anytime soon, but using the FQDN will help prevent any mailflow outages if our IPs do change.

In the Reflexion Portal.

Domains(To receive email)

Please put the original MX records received from Google into the Domains table and set delivery to port 25.  

Trusted Hosts

Please put the public IPs for GSuites into the Trusted hosts table.  You do not need to put in individual IPs, but the ranges published by Google will cover their IPs.  The list below contains most of their IPs, but please also check with Google in case new or different ranges are in use. (This list is accurate as of 23 October, 2019)

35.190.247.0/24         64.233.160.0/19
66.102.0.0/20             66.249.80.0/20
72.14.192.0/18           74.125.0.0/16
108.177.8.0/21           173.194.0.0/16
209.85.128.0/17         216.58.192.0/19
216.239.32.0/19         172.217.0.0/19
172.217.32.0/20         172.217.128.0/19
172.217.160.0/20       172.217.192.0/19
108.177.96.0/19         35.191.0.0/16
130.211.0.0/22

Addendum:
Please also remember to set your MX records to Reflexion.

https://community.sophos.com/kb/en-us/124513

While not mandatory, setting an SPF record can help streamline outbound mail transfer and helps our SPAM engine protect you and your clients from spoofed mail.

https://tickets.reflexion.net/index.php?/Knowledgebase/Article/View/10/1/reflexion-and-you-spf