Convert Earth Centered Earth Fixed (ECEF) coordinates to GSM coordinates

It is an urgent need to convert ECEF to GSM coordinates.

I tried converting ECEF to lat,lon,alt in geodetic cordinates using,

lla_gd = ecef2lla(X,Y,Z, 'WGS84'); % X,Y,Z are in ECEF cordinates
% the o/p lla_gd is in geodetic lat, lon, alt

Now i do not have any idea how to convert this geodetic lat, lon, alt to GSM cordinates.

Moreover, I tried converting geodetic lat,lon, alt to geocentric lat, lon, alt.

For this,

gc_lat = geod2geoc(lat, alt, 'WGS84');

the input lat and alt are in geodetic cordinates obtained by lla_gd

this gives only the o/p of geocentric latitude. There is no geocentric altitude o/p!

Please give answer. how to Convert Earth Centered Earth Fixed (ECEF) coordinates to GSM coordinates.

Help is highly appricaiated.

[Koundinya]

Altitude is measured from a point on the earth's surface, i.e , it is a topocentric measurement. So how and why would geocentric altitude even be defined ?