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.