I see no reference to either model on S-H's website. Are those UK- or Euro-spec'd units?
There is a GX3000 listed, which is a VHF/hailer; you can get the manual from the standard horizon website:
For the GX3000, it appears PA/Fog is selected from the front panel, so I don't think you can do it from the mic alone (standard mic). The unit accepts the RAM+ mic and the VH-310 radio telephone type mic, both of which have expanded controls. According to the manual, you can control the PA from these mic's alone.
Hope this helps;