It depends on the caller not on the language of the program being called. If you execute something via $PATH then most shells will only pass the command you typed and not the full path. Similarly, when you use a relative path like ./command then usually your argv0 will be that relative path and not the full path to the executable. So in practice argv may or may not be a full path and if it does not contain a slash then it generally isn't even a relative path (well, not relative to the current directory anyway).

For the help case in gp I think it makes sens for programs to always strip away anything up to including the last slash from argv0.

Languages might remove the dirname part, but argv[0] is not necessarily a path, it's just a string passed to the exec system call that is also passed to main(int argc, char *argv). While many languges don't call their main function that, somewhere in the runtime that's what it gets.