BaseCamp 4.3.2 doesn't support GPX 1.1 format

BaseCamp certainly understands GPX 1.1, but it will only read valid files.

Feel free to attach the file and people can help you figure out what's wrong with the file.

... your file is not valid.
The groundspeak extension is not declared. It also seems that you use the 1.1 version of the extension ("lastUpdated" is not available before). This version is still not released. You should not use it because no device understand it.
And some elements must be written in lowercase.

Simply check your files with a XML editor.

Sorry, looks like I shared one that was parsed by GPSBabel

https://www.dropbox.com/s/qe6tlr9gdnb2hkp/Overlay_Test.gpx

You haven't validated your file with a XML editor. It has still errors inside and you still use the v1.1. of the groundspeak extension. Basecamp won't regonize this as geocache.

- All groundspeak elements must be inside extension (<extensions> groundspeak.... </extensions>)
- <groundspeak:Owner id="0" /> should be <groundspeak:owner id="0" />
- <groundspeak:Attributes /> should be <groundspeak:attributes />

What ever you trying to do, it is better you use gpx 1.0 and the groundspeak extension 1.0.1