To identify which GKT version your Eclipse IDE under Linux is using go to Help -> About -> Installation Details and select the Configuration tab. Look there for
org.eclipse.swt.internal.gtk.version property. You can also use the “Copy to Clipboard” button so that you can copy the details into a text editor to search for it.
In my case it shows: org.eclipse.swt.internal.gtk.version=3.12.2 which obviously means that I’m using GKT3.
Thanks to Alexander Kurtakov for the tip.