If you're on macOS and are unable to download Marvin due to Chrome or another browser blocking it, you can download Marvin manually like this:

Step one: Open Terminal.  You can either press cmd+space and type Terminal, or find it in Applications → Utilities

Step two: Paste the following into the Terminal and press enter:

curl -L -o ~/Downloads/Marvin.dmg http://amazingmarvin.s3-website-us-east-1.amazonaws.com/Marvin.dmg 

Step three: Open your Downloads directory and double click Marvin.dmg.  You can then install as normal.

We have opened a support case with Google to determine why Marvin was flagged as dangerous.  If you are concerned you can try uploading it to https://virustotal.com, which scans for viruses.  Alternatively you can just use the web app, which has nearly all the functionality of the desktop app: https://app.amazingmarvin.com.

Stay safe!