PA28 INSTRUCTOR AND STUDENT LANDED AT THE WRONG ARPT THAT WAS TWR CTLED WITHOUT CLRNC.

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MY STUDENT AND I DEPARTED ADS WITH A CLASS B CLRNC TO RBD. AFTER COMPLETION OF A TOUCH-AND-GO AT RBD; WE REQUESTED TO PROCEED TO GKY. WE WERE TOLD TO PROCEED 'ON COURSE.' MOMENTS LATER; WE RECEIVED 'RADAR SVC TERMINATED; SQUAWK VFR; FREQ CHANGE APPROVED.' I WAS EXPECTING A HDOF TO GPM TWR. THE DFW VFR SECTIONAL DOES NOT DEPICT THE BOUNDARIES OR PRESENCE OF CLASS D AIRSPACE AT GPM. I WAS UNSURE; SO I CALLED GPM TWR TO INFORM THEM OF OUR LOCATION. GPM TWR REPLIED 'TRANSITION APPROVED;' THEN 'YOU MAY NOW CHANGE TO ARLINGTON FREQ.' AT THIS POINT; I ASSUMED THE ARPT DIRECTLY IN FRONT OF US WAS GKY CTAF AND PROCEEDED TO EXECUTE A 'TOUCH-AND-GO.' AS WE LIFTED OFF; I RECEIVED A SINKING FEELING WHEN I READ THE SIGN READING 'WELCOME TO GRAND PRAIRIE.' WE IMMEDIATELY TURNED AND PROCEEDED S OUT OF ANY POSSIBILITY OF D AIRSPACE. A MOMENT LATER; A MALE VOICE CAME OVER GKY FREQ AND SAID 'ACFT THAT JUST DEPARTED GPM; WHAT IS YOUR TAIL NUMBER?' WE DID NOT REPLY! QUESTIONS: BECAUSE THE CURRENT DFW SECTIONAL DOES NOT SHOW SEGMENTED BLUE LINES; IS IT CLASS D? WERE WE IN VIOLATION? IF IT IS; IN FACT; CLASS D AIRSPACE; DID OUR EARLIER CONTACT FULFILL THE REQUIREMENTS OF 'ESTABLISH COMS;' AND IF SO; DID WE NEED A CLRNC TO LAND?