i have received an e-mail from my website to my gmail SPAM box. I checked that e-mail source code SPF record, it shows: Result for IP address XY: NEUTRAL.

Received-SPF: neutral (google.com: is neither permitted nor denied by best guess record for domain of [email protected]) client-ip=;
I clicked for details and was brought to https://support.google.com/a/answer/33786 where they suggest to delete existing DNS TXT SPF record in my hosting account and create new with value "v=spf1 include:_spf.google.com ~all"
I do not understand why should i use their domain. Anyone knows?
I though i should use the domain or IP that is the one that relay the e-mail into the internet (my server public IP or address like mail.mydomain.com).

Actually my hosting provider says i have to use their domain in SPF in order e-mail is relayed thru their antispam system.