Add gcd to procedures in math module

Author: awvwgk

CHANGELOG.md

@@ -30,7 +30,7 @@ Full release notes available at [v0.1.0] tag.
   - new procedure `outer_product`
     [#432](https://github.com/fortran-lang/stdlib/pull/432)
 - new module `stdlib_logger`
-  - new derived type: `logger_type`
+  - new derived type `logger_type`
     [#228](https://github.com/fortran-lang/stdlib/pull/228)
     [#261](https://github.com/fortran-lang/stdlib/pull/261)
 - new module `stdlib_math`
@@ -40,6 +40,8 @@ Full release notes available at [v0.1.0] tag.
     [#420](https://github.com/fortran-lang/stdlib/pull/420)
   - new procedure `arange`
     [#480](https://github.com/fortran-lang/stdlib/pull/480)
+  - new procedure `gcd`
+    [#539](https://github.com/fortran-lang/stdlib/pull/539)
 - new module `stdlib_optval`
   [#73](https://github.com/fortran-lang/stdlib/pull/73)
   [#96](https://github.com/fortran-lang/stdlib/pull/96)