What does "gamma" in gamma-oryzanol mean?
No designations such as alpha-oryzanol, beta-oryzanol, etc. appear to be in use, and according to this WebMD entry, as well as this PubChem document, another name for gamma-oryzanol is simply "oryzanol".  

There seems to be some ambiguity as to just what oryzanol, or gamma-oryzanol refers to. According to Chemical Book:  


  Gamma Oryzanol is a mixture of 6 substances derived from rice bran oil, including sterols and ferulic acid, called Cycloartanyl Ferulate or Triterpene alcohol ferulate.  


But then other sources, including this US National Institutes of Health publication, consider gamma-oryzanol composed of four components:  


  There are four main components of γ-oryzanols, namely, cycloartenyl ferulate, 24-methylene cycloartenyl ferulate, campesteryl ferulate, and sitosteryl ferulate [original publication:  Lerma-García M. J., Herrero-Martínez J. M., Simó-Alfonso E. F., Mendonça C. R. B., Ramis-Ramos G. Composition, industrial processing and applications of rice bran γ-oryzanol. Food Chemistry. 2009;115(2):389–404. doi: 10.1016/j.foodchem.2009.01.063  


Further confusing the issue is that the PubChem article referenced above gives a single structure for gamma oryzanol (the same as that given in the OP), but does not include any physical properties that one would expect for a single pure compound (i.e. no melting point, solubility, vapor pressure data) although it does give the molecular weight. Again, PubChem lists "oryzanol" as a synonym and does not have any other oryzanol-named compound in it's database.

Summary TL/DR:
The gamma in "gamma-oryzanol" doesn't refer to a specific structure or isomer and there is no corresponding compounds named alpha-oryzanol, beta-oryzanol, etc. Rather, the term gamma-oryzanol is also simply called "oryzanol" and it refers to a combination of several different moieties as described above.