After emerging sci-geosciences/qgis-3.0.0, executing qgis in a terminal gives: Warning: Cannot open file '/usr/share/qgis/python/plugins/processing/images/ftools/vector_grid.svg', because: No such file or directory Warning: Cannot open file '/usr/share/qgis/python/plugins/processing/images/ftools/vector_grid.svg', because: No such file or directory So it seems like a file is missing in the installation. (I did not manually remove that file after installation.)
I can't see how this issue could be caused by packaging, so please report it upstream.
Please test with 3.0.1.
The warnings do not appear with QGIS 3.0.3.