Where should I place my global 'gitattributes' file?

Global vs. system-wide settings

There is some ambiguity in your question's terminology. In a Git context, "global" usually means "user-level"; in other words, a global setting affect all repositories for one specific user (the active one). In contrast, a system-wide setting affects all repositories for all users of a machine.

Repository-level gitattributes

(I'm only mentioning this for completeness.)

According to the relevant section of the Pro Git book,

If you wish to affect only a single repository (i.e., to assign attributes to files that are particular to one user’s workflow for that repository), then attributes should be placed in the $GIT_DIR/info/attributes file.

$GIT_DIR would typically expand to <path-to-repo-root-directory>/.git.

Global (user-level) gitattributes

According to the relevant section of the Pro Git book,

Attributes that should affect all repositories for a single user should be placed in a file specified by the core.attributesfile configuration option [...]. Its default value is $XDG_CONFIG_HOME/git/attributes. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/attributes is used instead.

You can also run the following command,

git config --global core.attributesfile <path>

to point Git to a custom path <path> for your global gitattributes file, e.g. ~/.gitattributes.

System-wide gitattributes

According to the relevant section of the Pro Git book,

Attributes for all users on a system should be placed in the $(prefix)/etc/gitattributes file.

which naturally begs the question:

[...] But where is $(prefix)?

See What is $(prefix) on $(prefix)/etc/gitconfig? for an answer. Unless you've assigned prefix a custom, non-empty value, $(prefix) expands to nothing by default; therefore, your system-wide gitattributes file should reside in /etc/.

If you read this far and still don't know where $prefix is (i.e. it's not just blank or /usr/local) or why your /etc/gitattributes is not being read, you can use strace or similar tools to see all the places git is checking. Strace prints a huge amount of debug information on stderr, so use 2>&1 and filter it, as I do below, with grep. The git command I chose for the example uses an attributes file because of the --stat which changes based on a -diff attribute, but any git command you are trying to run at the time should be fine.

For example, with an updated git installed on RHEL using SCL, you could end up with this:

$ strace -f git log --stat -1 2>&1 | grep --color 'open.*attr'
open("/opt/rh/rh-git218/root/usr/etc/gitattributes", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/username/.config/git/attributes", O_RDONLY) = -1 ENOENT (No such file or directory)
open(".gitattributes", O_RDONLY)        = -1 ENOENT (No such file or directory)
open(".git/info/attributes", O_RDONLY)  = -1 ENOENT (No such file or directory)

. . . and that shows that the $prefix here is /opt/rh/rh-git218/root/usr.

Strace's -f is not necessary here but if you don't know and don't care whether some command is going to fork, adding -f in the first place can eliminate a chance you won't see what you are looking for.