Converting converts the existing partition to GPT. It does not also expand the partition into the available space, you get to do that yourself.
open disk management and see if you can do it there (run/search for diskmgmt.msc) You can drag the right edge of the esiting partition only to the right. If you have available space to the left then you either need to use another utility or to copy the data off and reformat the drive and put the data back on.