Section 2.6 of the Getting Started Guide, "Getting Set Up in an IDE", is inaccurate in many places. Newcomers to the Grails framework that try to treat this as Gospel in selecting their IDE will be in for a very unwelcome and frustrating experience. They may also believe the fault lies with Grails itself, and not outdated support.
IntelliJ
The IntelliJ section states that "The community edition can be used for most things, although GSP syntax higlighting is only part of the ultimate edition". This is wrong, as trying to import any non-trivial project into the Community Edition will not work.
Eclipse
Here the GGTS is recommended. That might have worked for Grails 2 and below, but there's no way it's going to work with Grails 3. It doesn't even know of Grails 3 existence and will lead developers down a very frustrating road. There's a post here that details a possible approach with Mars:
but even this setup has been giving me some problems in my initial testing.
NetBeans
The documentation points to a link for setting up NetBeans. This document basically just says, "setup your Grails environment, install Netbeans, and you are good-to-go". Well that's not the case for me. I have a project with a src/ folder that contains some Java artefacts as well and, similar to Eclipse, Netbeans doesn't recognize these. There are other issues too.
Bottom Line: The only IDE that looks to work with Grails 3 is the paid edition of IntelliJ (which is too bad because it sets the price of entry to Grails development at $500 for the first year!). I think the documentation should reflect that as this is where users start development and you only get one chance to make a first impression.