Thanks for your response.
I copied all .js files from commonsjs-flat into node_modules\gsap.
import { SplitText, TimelineMax, TweenLite} from 'gsap';
No issues with TimelineMax and TweenLite.
But SplitText throws exception as follows:
__WEBPACK_IMPORTED_MODULE_1_gsap__.SplitText is not a constructor
Thanks for your help.