ASPiK uses the VSTGUI4 library. The ASPiK SDK includes a complete copy of the VSTGUI4 folder. You can also get it from GitHub, or the latest VST3 SDK. ALERT: the best option is to simply use the version with the ASPiK SDK. It will always be the proper version. We have found that the GitHub version is usually slightly different from the one included with the VST3 SDK – it is actually the “development” version.
As of this post, both the VST3 SDK version AND the GitHub version are marked as v4.9 but they are actually different, and the GitHub version will NOT work with the current ASPiK SDK OR the current VST3 SDK!
Note that your plugin will not compile without the VSTGUI4 library present, even if you do not need a custom GUI for your plugin. The individual API or universal API projects exist in an outer folder that we name myprojects and the outer VSTGUI4 folder named vstgui4 must exist in parallel with the plugin projects. See the SDK documentation or the video tutorials.
The current VSTGUI version for the ASPiK is: VSTGUI v4.9