Gatekeeping is trying to keep specific people out of a particular group or experience based on arbitrary, unimportant rules that the gatekeeper made up in their own mind. Like saying girls can't play with trucks because no penis or saying "if you like (x) musical artist, you're not a real fan of (x) musical genre". If you want a more denotative definition, it's the person that opens and closes the gate, and I see neither gates nor people keeping them in this picture.

Gatekeeping would be fining you for disclosing the location. Broadly speaking, gatekeeping is restriction that’s enforced by rules/customs or self enforced by someone without any consequences for not doing so. Kicking you a store for not buying stuff is gatekeeping. Not telling someone where you got a shirt is gatekeeping.

If you’re the type of person to be tagging the location previously, then you’re likely going to just tell everyone the location whether or not you adhere to the sign. And I’m assuming people get this information from their friends, but if it’s some random account like OP, then they’ll probably tell you as well. I cannot imagine a scenario in which someone would say “sorry bud but I can’t tell you where I hiked” just because a sign got put up. If someone says that, they’re gonna gatekeep with or without a sign.

Basically you’ll get the location from anyone you follow so there’s no one gatekeeping anything except an unenforceable sign