Please note that GeoTIFF can come in different special varieties and ImageJ is not a GIS. For some varieties
ImageJ might fail to open them.
However the Gdal library/software can open and convert most of them, see:
You can convert your image with gdal and then import the image in ImageJ. Most OpenSource GIS softwares using GDAL for reading and writing GIS formats.
GDAL can be used from the command line or using a GUI software.
Here is a nice link how to use it via command line (and more):
By the way, as you are familiar with R the library rgdal is a wrapper for the GDAL library which can be used to open georeferenced images with R.
I’ve written a simple BeanShell script in Bio7 which uses rdal to open GIS images. It also transfers the images to the ImageJ view, see here for example:
With the first action you should at least get some information about yout file format.