This tool provides a way to query all IPs related to the SPF entry of a specific domain:

$ get_spf_ips gmail.com
172.217.0.0/19
216.239.32.0/19
216.58.192.0/19
209.85.128.0/17
207.126.144.0/20
173.194.0.0/16
108.177.8.0/21
74.125.0.0/16
72.14.192.0/18
66.249.80.0/20
66.102.0.0/20
64.233.160.0/19
64.18.0.0/20

This is done by recursively querying the TXT entries of the domain. For example, you can see that the first IPs printed above are extracted from something like the following:

$ dig TXT gmail.com | grep 'v=spf1'
gmail.com.              225     IN      TXT     "v=spf1 redirect=_spf.google.com"

$ dig TXT _spf.google.com | grep 'v=spf1'
_spf.google.com.        161     IN      TXT     "v=spf1 include:_netblocks.google.com include:_netblocks2.google.com include:_netblocks3.google.com ~all"

$ dig TXT _netblocks.google.com | grep 'v=spf1'
_netblocks.google.com.  553     IN      TXT     "v=spf1 ip4:64.18.0.0/20 ip4:64.233.160.0/19 ip4:66.102.0.0/20 ip4:66.249.80.0/20 ip4:72.14.192.0/18 ip4:74.125.0.0/16 ip4:108.177.8.0/21 ip4:173.194.0.0/16 ip4:207.126.144.0/20 ip4:209.85.128.0/17 ip4:216.58.192.0/19 ip4:216.239.32.0/19 ~all"

get_spf_ips is known to work for gmail.com and outlook.com.

NOTE: As of this writing this tool does not provide sorting and deduplicaton of IP addresses. Also note that only IPv4 addresses will be provided.