Apple ID email address clarifications

Hello,

I've been an iPhone user since the original iPhone and now found myself with some problems regarding the online services.

As an introduction, I've registered my Gmail e-mail address as my Apple ID and had no problems at all with it until now.

It happens that whenever anyone tries to add me to Find My Friends or iMessage using my e-mail address, the system doesn't recognize it as if it were a valid Apple ID. However, upon checking the Manage my Apple ID page, I see that my e-mail is properly configured as both my Apple ID and primary e-mail, and I also found an alternative e-mail under the @me.com domain.
Now, if you try to add me as a contact to Find My Friends using my Gmail address, a grayed question mark appears next to it. However, if you use the @me.com address, everything goes well.

I've tried removing all alternative addresses but the @me.com one keeps returning.

Is there anything I can do to fix this? I really don't want to use the @me.com email address at all.

Thanks!
Nico

PS: the question is not how to delete the @me.com address, but why is my Gmail one not working properly with the online services.