i'm a student . Does anyone know where to find pseudo random number generator source code in VHDL for Gain_128 and w7? thanks

If I google gain_128, w7, and pseudorandom number generator I get this request. I have therefore no idea what you are requesting.

If you just want a random number generator, there's a series of three articles about creating a linear feedback shift register on zipcpu.com. The first article discusses the traditional LFSR implementations, the second gives an example LFSR, and the third article discusses how to generate multiple bits from an LFSR at once.

Admittedly, all of the implementations are in Verilog. However, at least the last one comes with a formal proof that it works, in case formal verification is something you are interested in.
Dan