Why not use pattern of GOF gang of four

The standard and mostly recognized pattern to be followed for SRS, is that specified by IEEE.   So why should anyone follow some other pattern dictated by some other GANG...GOF

Yes. Standard is defined by IEEE. where GOF is collection of design pattrens which can be followed. But these too, does not restrict the design pattren choice. Choice of design pattren is dependent upon requirements of a system. Even if IEEE standard is not followed, it does not make harm. Design pattren can be chosen from GOF and even design pattren can be hybrid or customized depending upon need.