ModularUI 2

ModularUI 2 is the UI library used in GTM starting from 8.0.0, and is the successor to LDLib UI. From ModularUI 1.12's github:

With ModularUI you can build GUIs fast by adding Widgets to panels with layout widgets, so you don't have to calculate positions and sizes yourself. ModularUI is very dynamic and allows for very complicated client-server synced GUIs.