The Xilinx datasheet said the Global clock should be input to GCLK pin, but when I want to output a global clock to another chip, shall I link the ouput wire to a GCLK pin?
I think that every input or output can be considered as output clock. your clock is used as next stage input clock, so you can choose any IO.
In my experience, input clock must not be global clock in common ocassion.
Yes input clock need not always be GCLK pin. Using GCLK pin reduces the skew between the pad and the clock buffer inside the device. You can also use any other input pin for clock.