Can I flush, or release, my GPU memory?
There has often been problems with AMD based GPUs and their reporting of video memory usage in iStat Menu. This has in the past been for example always reporting 100% usage, always reporting a climbing usage, etc.

If you haven't got any performance degradation, and your computer usage really isn't affected by this - I would just ignore the readout. 

There's no tool to do a manual release - it doesn't really make sense. If the VRAM is actually in use, "manually releasing" it would make programs crash or behave unpredictable - nobody wants that.

If you really want you could probably lower the VRAM usage by logging out, or by killing WindowServer in its entirety. There's just no real reason to do so - unless you're experiencing a performance degradation, and then it would be better to analyze that specifically.