Its not something specific to Phonegap, rather it's specific to the underlying system web view in use. As such, "caniuse.com" is a really good indication of support. http://caniuse.com/#search=webgl indicates that WebGL is supported on iOS 8 and higher.
Note that there are edge cases in how your code may work in PG Developer vs a build of your app using the CLI/PGB. This generally only applies to specific use cases with plugins (like the File or Media plugin), but it wouldn't hurt to do an actual build of your app and see if it exhibits the same problem.
Unfortunately, I'm not terribly familiar with webgl, nor do I know what tutorials you are following for it, so I can't offer much more help.