GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it
DeclareGlobalFunction("SL2ZmElementsDecomposition");
DeclareGlobalFunction("SL2ZTree");
DeclareGlobalFunction("ResolutionGTree");
DeclareGlobalFunction("TreeOfResolutionsToSL2Zcomplex");
DeclareGlobalFunction("SL2ZResolution");
DeclareGlobalFunction("SL2ZResolution_alt");
DeclareGlobalFunction("ConjugateSL2ZGroup");
DeclareGlobalFunction("CongruenceSubgroup");