Outlook Client can't send gmail messages from @outlook

This is kind of confusing to explain, and because of that I can't find anything online about it. Years ago I forwarded my gmail account to an outlook account and then I could set the @gmail.com address as the default "from" email address when using outlook, both online and through the client (Outlook client from Office 365 and using Windows 10).

Example: I receive an email at address@gmail.com which was forwarded to address@outlook.com. I could then respond from outlook, both web and desktop clients, and the email would be sent back as though from address@gmail.com. So I didn't need to update people to the new address, I could just keep sending stuff as though it was from my gmail and even I am using an outlook email.

Sometime over a year ago it stopped letting me do this from my desktop outlook client. It says that I don't have permission to send on behalf of address@gmail.com and I have to resend the email from address@outlook.com, which has caused people to start asking me what my real email address is.

If I send from the outlook web client, I can still send with address@gmail.com as my default address, but I can't from the desktop client.

I need help doing one of two things. Either getting the desktop client to allow me to send as though the from address is the gmail address, or I need it to stop making me manually change every email as being from address@outlook.com in order to send.

Any help would be appreciated. If my explanation isn't clear, please feel free to ask clarifying questions.

As a note, the issue is on both my desktop and laptop, so it isn't limited to one device.

Not too long ago I had an issue with GMail blocking access from what it considered 'insecure apps'. There was a setting in GMail options to allow this-once I turned it on the problem went away. I'm not sure that it's the same problem but you might give it a try.