Since they weren't specific, I'd sent an email formed as a cover letter and resume in an attachment (with a note in your cover letter that if the person has any problems with the attachment, you would be happy to send the resume in the body of an email).
My one concern is that this is going to a gmail address...which would imply that this could be something as simple as a small/personal or home business, or something bad like a scam or questionable in nature. Hard to tell without the body of the job description (though it maybe it's just me and years of writing these things, I'm going to assume most most people find it pretty easy to spot when they're questionable or more sleazy/crooked type of positions
). But personally, I wouldn't immediately ask what company this is for, as if they are professional and interested, they will fill you in with a return email. Just proceed with caution.