I am not familiar with Xilinx library but if you needs Additive Gaussion White Noise most probably you will be able to do it using simple PRBS folowed by the low-pass digital filter and finaly decimator.
There is the suprenum probability theorem that tells that
a large sum of probable numbers has Gaussian distribution.
I have designed such a Gaussian generator in VHDL
which consists of uniform noise generator based on shift registers
and adder which adds more than 8 samples from that generator.
Then the adder output samples have practically Gaussian distribution.