I found this detail in STAAD.Pro 2004 software release report (build 1002.India) about GLD parameter:
<Quoted from the report in description of GLD parameter for IS13920 design>
Gravity load number to be considered for calculating equivalent udl on span of the beam, in case no EUDL is mentioned in the input. This loadcase can be any static loadcase containing MEMBER LOAD on the beam which includes UNI, CON, LIN and TRAP member loading. CMOM member loading is considered only when it is specified in local direction. FLOOR LOAD is also considered.
The load can be primary or combination load. For combination load, only load numbers included in load combination is considered. The load factors are ignored. Internally, the unfactored load is multiplied by a factor 1.2 during design
If both EUDL and GLD parameters are mentioned in the input mentioned EUDL will be considered in design
No dynamic (Response Spectrum, 1893, Time History) and moving load cases are considered.
CMOM member loading in global direction is not considered
UMOM member loading is not considered.
<End of Quoted text>
So, load combination can be used as per above detail.