Pretty much only covers license holders
§ 30.06. TRESPASS BY HOLDER OF LICENSE TO CARRY CONCEALED
Section 30.06 of the Texas Penal Code spells out what is criminal trepass by a CHL. It says that if you had written or oral notice that entry was forbidden based on your carrying a firearm, then you have committed criminal trespass. It goes on to describe the specific language that must be used in written communication in order to be considered "notice" to CHLs.
Section 30.05 of the Texas Penal Code spells out what is criminal trespass for everyone else. Under that section if:
(1) had notice that the entry was forbidden; or
(2) received notice to depart but failed to do so.
then you are guilty of criminal trespass. Notice is defined as "(A) oral or written communication by the owner or someone with apparent authority to act for the owner" (as well as several other methods of notice that aren't applicable to this discussion).
Unlike Sec. 30.06, there isn't any specific requirement for written communication, so presumably, ANY written notification that meets the criteria would be effective notice. However, I don't see how you could possibly walk into court and say "I saw the 30.06 sign; but I am not a CHL holder so I didn't have effective notice that entry was forbidden."
If the sign meets the 30.06 requirements, I would be willing to bet a court will consider it effective notice to someone with a firearm in the car without a CHL.