What's the easiest way to save a GIFV as a simple GIF?

GIFV isn't a GIF at all, in the traditional sense anyway. It's actually a video file.

Here's what I read from Imgur's blog post introducing GIFV:

The cornerstone of Project GIFV is a platform-wide upgrade to automatically convert uploaded GIF files on the fly into the WebM or MP4 video formats, depending on browser support.
...
Project GIFV implements video in a way that looks and behaves exactly like a GIF.

If you right-clicked on a GIFV and selected Open link in new tab (or a similar option) you'd see that Imgur creates a minimalistic web page that it seems like you just opened a GIF in a new tab but right-click anywhere on the page and select View Source and you'll see that isn't the case.

To answer your question, the easiest way to save a GIFV as a GIF (if it is hosted on Imgur.com) is to open it in a new tab and click on the download link at the bottom.

If the GIFV is hosted on another site (not that I know of any other sites hosting GIFV) you can use either of the two online video conversion services:

1. MP4 to GIF
2. WEBM to GIF

NOTE: Downloading a GIFV as a proper GIF will most certainly increase the file size by a big margin. If you'd rather download the WEBM or MP4 video instead, you may right click on the GIFV and select Save video as... (or a similar option).

For Imgur URLs, if you remove the V from .gifv then you will get the original animated GIF.

(This might not be "a simple GIF" but could be a large file, as explained on Imgur's blog. Like in http://i.imgur.com/zvATqgs.gifv the embedded video is 3.4 MB, but the original animated GIF http://i.imgur.com/zvATqgs.gif is 46.7 MB.)

Renaming .gifv to .gif extension on the address bar does not work for me.

Right click on the .gifv and copy video url, this will copy it's url with .mp4 extension.

Use wget or other tools to download the file. It usually redirects back to the .gifv link on a browser.