Use “include:aspmx.googlemail.com” in your SPF record:

yourdomain.com.		300	IN	TXT	"v=spf1 a mx  include:aspmx.googlemail.com -all

Check your /var/log/mail.log to make sure it’s working as expected:

Aug 10 08:30:05 pizzapazzi postfix/smtpd[23061]: connect from mail-ob0-f199.google.com[209.85.214.199]
Aug 10 08:30:12 pizzapazzi postfix/policy-spf[23074]: : SPF pass (Mechanism 'include:aspmx.googlemail.com' matched): Envelope-from: xxxxxxxxxx@yourdomain.com
Aug 10 08:30:12 pizzapazzi postfix/policy-spf[23074]: handler sender_policy_framework: is decisive.