Developers will be the first to sneer at the exchange-email-for-a-url lead magnet because they will (rightly) point out: you don't need my G-D email to download a file.

What might make sense to be gated by email?

• email-based courses
• open source boilerplates
• membership signup
• web-based content
• platform content (like Gumroad)

A downloadable file isn't the worst thing you can do but there are plenty of other ways that will seem less... lazy.