To use the TCK pin as a global I/O pin, it is not sufficient to just set the bit in the GLBEN register. It is also necessary to configure the appropriate bitfield in the IOMUX register to configure the TCK bit as something other than JTAG. Otherwise edges on the TCK bit may cause unpredictable behavior in the JTAG logic.
Note that the IOMUX has no effect on which pin the GLBIO signal goes to.