While it's probably safe (who knows, right?), I still wouldn't recommend it because a lot of people are paranoid about it, resulting in a lower response rate.
I also think emailing from the email attached to the domain you are trying to guest post from looks more professional.
Maybe other people react differently, but personally, I rarely even read guest post requests coming from gmail accounts anymore because they are usually spammy mass-emailed guest post requests from spammers. Any website with a shred of credibility is going to be using their legitimate website email address, not some free email service which they only signed up for to mass-email spam requests.
I get so many of these requests from gmail accounts (almost all of which are spam) that I don't even bother looking at them anymore.