I wonder if you might have some inbox rule that blocks the entire gmail.com domain.
Make sure you do this on a computer, not on smartphone or tablet. Go to https://www.outlook.com and log in, click on the gear icon on the top right to launch the settings.
Then, on the Mail tab, go to Rules. Are there any rules set up here? If yes, delete them all.
Then check the conditional format, again, if it's there, delete it.
Also, if you check your outlook.com address on your iPhone (since you chose the iOS tag), you might want to go to your iPhone's own blocked contacts settings to see if any gmail block is listed there too.