After reviewing this, https://gerrit.wikimedia.org/r/#/c/330084, I think many messages (probably all) could use GENDER, especially with the ones that have you/your or some other variant in the message.
For example,
invite-sent, invite-back-to-userpage, invite-your-friends, and many more could use it.
I would be interested in working on this should it be a valid task @SamanthaNguyen @ashley
Thanks!
I don't believe they can be, as the messages are talking directly to you, and not referring to someone else. This is you instead of him/her/their and they're already concise and gender-neutral. :) If you want to add it for other languages that rely on gender pronouns much more like Spanish or French and willing to add the associated i18n files along with using GENDER in the messages, go ahead, although English doesn't require it.
I'm going to go ahead and mark this as declined, but thanks for the suggestion. :)
Edit: Also do note this section on the "Localistation" article at mw.org as a reference, along with the last line:
When a message talks about a user, or relates to a user, or addresses a user directly, the user name should be passed to the message as a parameter. Thus languages having to, or wanting to, use proper gender dependent grammar, can do so. This should be done even when the user name is not intended to appear in the message, such as in "inform the user on his/her talk page", which is better made "inform the user on {{GENDER:$1|his|her|their}} talk page" in English as well.
This does not mean that you are encouraged to "sexualise" messages' language: please use gender-neutral language whenever this can be done with clarity and precision.