You can download TextPad 5.2 and use it as an Integrated Development Environment (IDE) by doing the following.
Download the syntax file
The syntax highlighting file gridlabd.syn should be saved in the system folder of TextPad 5.
Configure the run command
You can add the GridLAB-D executable (wherever you installed it) and configure a macro item to run it using the Configure/Preferences menu item. Figure 1 illustrates how this dialog is often set up. The macro item will be assigned to the Ctrl-1 key if it is the first one listed, Ctrl-2 as second, etc.