why only transmit your altitude and nothing else such as lat/long, spd, callsign.
What kind of aircraft do you see sending only altitude? I assume those are helicopters or light aircraft that don't have other information than altitude available to the transponder. In such case the transponder sends out altitude only on ADS-B (and maybe a few additional status messages).
Flights over the Gulf are required to have ADS-B.
The
ADS-B-OUT rule for the US (include class E airspace over Gulf of Mexico) was accepted on May 27, 2010 and effective since August 11, 2010 but the compliance date is January 1, 2020. Until then you can expect to see non-compliant aircraft.