You need to give it a name
typedef union
{
uint8_t All;
struct
{
uint8_t G:1;
uint8_t O_B:1;
uint8_t Unused:1;
uint8_t W1:1;
uint8_t W2:1;
uint8_t Y1:1;
uint8_t Y2:1;
uint8_t IAQ:1;
} bit_field;
} _UNION_VARIABLE;
and then you can go
_UNION_VARIABLE.bit_field.G;