It's been standard practice for a very long time to use the Gradle wrapper, which fixes the Gradle version, so there really shouldn't be a problem building an old project.
You're right that including the Gradle wrapper is a common good practice for years. But if you have to or want to move to a newer Gradle version than you can enjoy the full randomness of the domain specific language from Gradle itself and from a thousand plug-ins.
Exactly, if you have a very old Gradle project, it's best to use the wrapper to keep running with the version of Gradle it was written for, and make sure to use a compatible JDK to compile with (older Gradle versions won't work on new Java versions, which goes against most Java code which will almost certainly run forever even if written for Java 1.1).
Making changes to one is indeed fraught.