Keep in mind there are three modules: grails on the backend, apollo plugin / Jbrowse, and GWT depending on what you want to change.
Can Apollo run with Groovy 3?
I *think* this is immaterial as Grails (2.5.5) will use its own version of groovy for building. Regardless, if you need to switch versions in the groovy universe I would recommend: https://sdkman.io/install which handles groovy, gradle, grails versions.
Which IDE do you use?
I use IntelliJ ultimate.
You could also get away with NetBeans or Eclipse, but I highly recommend IntelliJ.
I’d be willing to a developer meeting if any others (or just you) were interested.
Either way, let me know if you need any help / pointers.