I don't think you can get down to that level of granularity with an IPS exception; the best you could do is set up an exception for their MX 11.11.11.148 (and any other ones they have) and then specify the "Suspicious Mail Attachment Containing JavaScript Code" signature.
You can determine their inbound MX systems yourself as shown below; bear in mind these may not necessarily reflect what their outbound MX systems will be, but it is a good starting point:
# nslookup
Default Server: unknown
Address: 172.31.128.1
> set type=mx
> gmail.com
Server: unknown
Address: 172.31.128.1
Non-authoritative answer:
gmail.com MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google.com
gmail.com MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google.com
gmail.com MX preference = 20, mail exchanger = alt2.gmail-smtp-in.l.google.com
gmail.com MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com MX preference = 30, mail exchanger = alt3.gmail-smtp-in.l.google.com
alt1.gmail-smtp-in.l.google.com internet address = 173.194.77.26
alt2.gmail-smtp-in.l.google.com internet address = 173.194.219.27
gmail-smtp-in.l.google.com AAAA IPv6 address = 2607:f8b0:4001:c16::1b
alt3.gmail-smtp-in.l.google.com internet address = 172.217.197.27
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