Bratio

Generation of a basis is actually controlled by the setting of Bratio.

GAMS will suggest a basis as long as the number of candidates for basis inclusion exceed the number of needed elements times the value of Bratio.  Thus in a 1000 equation LP with the default value of Bratio (0.25) GAMS will not suggest a basis unless it could find at least 250 elements to include in it.  Generally this would not be the case unless the problem had been previously solved and then all 1000 elements would be known or other efforts are made.  This means a basis is always suggested by default as long as I are dealing with a second or later solve.  It also means that by setting Bratio to 1.0 one can cause the basis to never be suggested and by setting it to a small number (0.01) one can cause GAMS to use whatever is available.