Gadget means an object having User Interface. Packages - java.awt, java.applet, and javax.swing are for the same purpose. You may use existing gadget or create new one.
Gadgets are light-weight single-purpose applications that can sit on the user's computer desktop, or are hosted on a web page. According to Microsoft, it will be possible for the different types of gadgets to run on different environments without modification, but this is currently not the case.
Gadgets can be an applet, or any other GUI component. If poster want to create gadgets then he/she should use java.awt, javax.swing and other packages.
Netscape java plug-ins is also environment to host gadgets.