Hi. I'm Brian and I'll try to help.
You can simply add the "gmail.com" domain to your Safe Sender list if you want all mail with a From address ending in @gmail.com to flow to the Inbox. Of course, this means that you'll also receive any real spam that uses a Gmail From address into your Inbox as well. Outlook.com does not have a blanket "treat all Gmail as spam" rule. I get Gmail messages all the time.
Visit https://outlook.live.com/mail/options/mail/junkEmail to see if you have the "Strict" radio button selected. If you do, then anyone sending mail to you whose address is not a contact or in the Safe Sender list will have their messages delivered to the Junk Email folder. Choosing Standard will ease that restriction so that you will be able to receive most new address email.
If the messages are not arriving in the Junk Email folder or the Inbox (that is, they aren't being received at all), adding the From address to the Safe Sender list cannot help. The Safe Sender list only allows messages that would normally be delivered to the Junk Email folder to instead be delivered to Inbox.