Gunslinger's OFP Editor Update (EU) is a huge addon to the OFP mission editor. The EU adds support for nearly 500 new objects. You can find out more about the EU by reading the EU's README file.
If you've got OFP version 1.75 and above, you'll need Kegetys' newer Editor Addon, which is a complete replacement for Gunslinger's original EU. The latest version of Kegetys' Editor Addon, 1.11, can be downloaded from here. For OFP version 1.46 and below, you'll need the Gunslinger original. The latest version, 1.02, can be downloaded from here.
Now, the big question is "is it safe"? The answer is that, using OFP version 1.75 and above with Kegetys' Editor addon, there are no reported side effects. However, that will only be true if you do not have any older EU related addon or patch files in your ADDONS subdirectory at the same time.
As for OFP version 1.46 and below with Gunslinger's EU, there were reports of various problems showing up in OFP that disappear after the EU has been uninstalled. The most common problem reported as being associated with the EU is missing tank HUD data. Does the latest Gunslinger EU version correct these problems for OFP 1.46 and below? I'm not sure. Personally I had the EU installed for quite some time back in OFP 1.46 and never experienced any noticeable problems. Your mileage may vary, of course.