We need to run both the application serprately and continue your developement.
These command can be used to run these application while development.
Root directory: ./gradlew bootRun
Frontend directory inside root: npm start

Once development completed and ready for deployment, update your build.gradle file to build react app as well during this.

plugins {
    id 'org.siouan.frontend-jdk11' version '6.0.0'
}

frontend {
    nodeVersion = '16.1.0'
    packageJsonDirectory= file("${projectDir}/frontend")
    nodeInstallDirectory = file("${projectDir}/frontend/node")
    assembleScript = 'run build'
}
*Above task will do the react app build for you
Now we need to copy this build to our spring application build folder

task copyReactApp(type: Copy) {
    from "${projectDir}/frontend/build"
    into "${projectDir}/build/resources/main/public/."
}
bootJar.dependsOn(copyReactApp)

And you are done with gradle changes to be done. Next gradle build will give you jar file with react application in it.