When refining with HighScore Plus, inserting the CIF file for gamma-Ca2SiO4 and then refining it will show "unusual space group setting for phase: gamma-Ca2SiO4". This occurs for all gamma-Ca2SiO4 CIF files. It is confusing to me.
Please attach the respective CIF file for such a question. Without that information nobody can help you.
A wild guess: since gamma Ca2SiO4 should be orthorhombic as in a phase transition from the monoclinic beta phase. The CIF file might just have the gamma phase in a monoclinic setting as well with the c-axis as unique axis instead of the standard monoclinic b-setting.
Check out the usual crystal system descriptions in any text book on crystallography.