As stated in your question, UN 2795 Batteries, wet, filled with alkali do not have a PG rating but according to PI 870 and SP A802, the packaging need to satisfy PG II performance. It will be necessary to check the Test Certificate of the package in question. The Test Certificate sometimes will indicate that it can accommodate so many kilograms for PG I (X) substance, so many kilograms for PG II (Y) substance and so many kilograms for PG III (Z) substance. The rough relation between PG I and PG II is 1.5 times.
So, if the Test Certificate certifies it can hold 24.3kg for PG I substance, it may carry a remark that it can hold 36.45kg or 36kg if used for PG II substance. It is then insufficient to contain 54.5kg. If the Test Certificate do not carry such extra remarks, the package then is only useable for 24.3kg whether used for PG I, PG II or PG III substances. You will have to check the Test Certificate. Furthermore, the detailed specification and requirement for a 4GV is written in DGR 18.104.22.168 in details.