Historically IJA had 34 "double strength" divisions (equal to 68 ingame divisions if we go strictly by double, reality was ~1,100,000 men or 110 ingame divisions). This ignores the IJN which also participated in the Battle of Shanghai for example (heck, none of the units that participated in the Battle of Shanghai on the Japanese OR Chinese side exists in game, instead we have crappy ahistoric "baoan" militia divisions) Reference: Wikipedia
What Japan has in game:
-25 Outdated Infantry (more outdated than CHI in some cases)
-6 Cavalry
-31 Garr
-3 HQ
Obsolete Divisions: 46
Obsolete Brigade: 10
I wonder why 25 outdated 1926 infantry/1921 garrison (that all have basically no strength) are failing to fight properly against 100+ 1921 infantry that instantly reinforce upgrade to 1931/1936....*/sarcasm.
The simplest and most accurate thing is not to nerf China's GDE but to buff Japan's laughably weak army.
Sino-Japan Japan Strength: (from wiki)
3,900,000
900,000 Chinese Collaborators
= 480 divisions total of units. I highly doubt Japan can come close to this amount in game (just in the sino-japan theater), even with reinforcement.
Sino-Japan China Strength:
5,600,000
3,600 Soviets (1937–40)
900 US aircraft (1942–45)