Re: others forging your domain by using Gmail - this is only true in narrow edge cases like this one (which appears to have been relatively quickly fixed by Gmail).

In practice, the Gmail and G suite UIs provide enforcement mechanisms to ensure you own a domain/address before allowing you to send from it.

Does it potentially open you up to spoofing if another loophole/exploit like this is discovered? Sure. But as risk factors go, depending on who you are, that's probably relatively low on the list.