If the GATE is pulsed during counting with TGS violating the spec but TGH meeting the spec, the initial count will be loaded on either the first or second falling edge of CLK (A or B in Figure 2). In this case, the internal logic can resolve the state of the GATE, but not in time.
The internal logic provides a way to ensure recognition of the GATE on the rising edge of the CLK. In Modes 2 and 3, the 8254 is both edge and level triggered. In the second case, where both specs are violated, neither the edge nor the level detector will detect the GATE going high, therefore, the count may not be loaded. As long as TGH is valid, the level detector will detect GATE going high even though the edge detector doesn't. This allows count to be loaded on either the first or second count, but it is not guaranteed on which falling edge, A or B (Figure 2), the loading will occur.