Also you have to switch over you nameservers to cloudflare before it will give you the option to transfer a domain to them.

and you also have to set spf records to enable "Send mail as:" from gmail. [1][2] (something like v=spf1 include:_spf.mx.cloudflare.net include:_spf.google.com ~all )

[1]https://community.cloudflare.com/t/solved-how-to-use-gmail-s...

[2]https://support.google.com/a/answer/10685031?hl=en