Find another issue with the font of annotation of colorbars:
I tried to set the font larger but it did not work (in GMT6.5):
gmt set FONT 12,Times-Roman
gmt set MAP_ANNOT_OBLIQUE anywhere
gmt begin gravity png
gmt grdimage @earth_faa_01m -JOb168/51/172/32.5/14c -R-500/2300/-400/400+uk -BeNsW -Bxa5f1 -Bya5f1
gmt colorbar -C -Dx14.2c/2c+jML+w3c/0.2c+mc -Ba100f50 -By+lmGal
gmt colorbar -C -Dx7c/-0.2c+jTC+w6c/0.2c+mc+h -Ba100f50 -By+lmGal
gmt end show
It works well in GMT 6.4.