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The process of obtaining a SIMD (Single Instruction Multiple Data) subroutine in gfortran involves the following steps:

1. Choose the appropriate SIMD instruction set for your target hardware. For example, SSE (Streaming SIMD Extensions) is commonly used on x86 processors.

2. Write the subroutine in standard fortran, using intrinsic functions or statements such as "DO" loops that can be vectorized by the compiler.

3. Use compiler directives to indicate that the subroutine should be vectorized. This can be done using the "#pragma simd" directive on some compilers or the "ATTRIBUTES VECTOR" directive in gfortran.

4. Compile the subroutine using a compiler that supports SIMD vectorization. For gfortran, the "-O3" flag can be used to optimize for performance, and the "-ftree-vectorize" flag can enable vectorization.

5. Test and optimize the subroutine for performance, making adjustments as needed to improve vectorization and minimize overhead.

Note that the specific steps and details may vary depending on the target hardware and software environment. It is important to consult the documentation and resources for your specific compiler and platform to ensure the best results.