The answer is not always.
OFP guru Kegetys has therefore revamped Gunslinger's EU to make it 100% compatible with OFP: Resistance. He's wrapped it all up into what's known as Kegetys' Editor Addon. You can download it from Kegetys' OFP site, from Kyllikki's Local Defense Detachment or from here at The FAQ.
Make sure you read the README file that comes with Kegety's Editor Addon. Among other things, Kegetys states there that it is recommended to remove Gunlsinger's EU file, EditorUpgrade102.PBO, before installing Kegetys' Editor Addon in order to avoid conflicts.