We could store version information in any jar manifest file using Implementation-* attributes:
Manifest-Version: 1.0
Ant-Version: Apache Ant 1.8.0
Created-By: 1.6.0_20-b02 (Sun Microsystems Inc.)
Implementation-Title: GWT User library / GWT Dev library / Elemental / ... / ....
Implementation-Version: 2.7.0
This can then be read in Java using classFromJar.getPackage().getImplementation[Title | Version | Vendor](). If all GWT jars would have that information you can choose whatever jar you like most.
-- J.