Can I use GNU ps on OS X? We can install GNU coreutils via brew. But there's no ps in the utils. Can we use gnu ps in OS X?

I'd like to use GNU ps as it has more options and features.
There is no portable ps program. The ps command available on linux is, as others have mentioned, from the "procps" package. The reason why this cannot be ported to OSX is because Linux and OSX kernels do not expose this information in the same way. Linux uses a pseudo-filesystem in /proc, whereas OSX uses the sysctl function. Other systems may use either mechanism but provide data in a different format, or may require programs like ps to read directly from the kernel's memory.

In general there is no standard for how process information is available to programs like ps and top; so the program has to be designed for a specific operating system. If there is a specific feature missing from the OSX ps, you'll have to find another program that can do it, or write one yourself (look at the sysctl manual, in particular KERN_PROC as a starting point), or modify the existing ps command to add the feature.

Alternately, if you can get what you need by parsing the output of the ps command itself, you may be able to write a portable program - the output with the -o option is reasonably reliable across platforms, particularly if you refer to the UNIX standard for the column names to use.