According to DGR Table 3.6.B the following criteria must be met to be categorized as PG I. Firstly, LC50 must be below 1,000mL/m3. The substance in question is 300mL/m3 so it satisfies this criterion.
Saturated vapor concentration should be more than 10 X LC50. The substance in question is LC50 300, therefore 10 X 300 is 3,000 and would also satisfy the other criterion.
Thus, this substance is PG I.