You should first look up what a Sender Policy is. It is up to you to decide, which servers may send e-mail on behalf of your domain, all other servers will be suspicious (usually spammers).
If you want to send e-mail only from GMail, you need to find and copy GMail's policy:
piotr@bialykiel:~$ dig gmail.com TXT +short
"globalsign-smime-dv=CDYX+XFHUw2wml6/Gb8+59BsH31KzUr6c1l2BPvqKX8="
"v=spf1 redirect=_spf.google.com"
piotr@bialykiel:~$ dig _spf.google.com TXT +short
"v=spf1 include:_netblocks.google.com include:_netblocks2.google.com include:_netblocks3.google.com ~all"
So probably what you want is a TXT record containing:
"v=spf1 include=_spf.google.com ~all"
It means: we usually send from these IP addresses (you can find a list digging further the includes), but we may use another IP address.
E-mail providers usually explain what records are needed. You'll find Google's explanation on their help pages.