これはspringBootプラグインのドキュメントからのものである:
The gradle plugin automatically extends your build script DSL with a
springBoot element for global configuration of the Boot plugin. Set
the appropriate properties as you would with any other Gradle
extension (see below for a list of configuration options):
以下bootRepackageプラグインのmainClass素子の構成の一例があること:すなわち
The main class that should be run. If not specified, and you
have applied the application plugin, the mainClassName project
property will be used. If the application plugin has not been
applied or no mainClassName has been specified, the archive will
be searched for a suitable class. "Suitable" means a unique class
with a well-formed main() method (if more than one is found the
build will fail). If you have applied the application plugin,
the main class can also be specified via its "run" task
(main property) and/or its "startScripts" task
(mainClassName property) as an alternative to using the
"springBoot" configuration.
これら二つが同一です。