I just did dumps of both DLLs and GDI32.dll does not import anything from MSIMG32.DLL, but here is a listing of what MSIMG32.dll imports from GDI32.dll:
GDI32.dll
6B2C1000 Import Address Table
6B2C15C8 Import Name Table
FFFFFFFF time date stamp
FFFFFFFF Index of first forwarder reference
77F6763F ED GdiGradientFill
77F677F1 109 GdiTransparentBlt
77F4334B 157 GetObjectA
77F45946 12D GetDeviceCaps
77F67553 B6 GdiAlphaBlend
77F420CB 158 GetObjectType
77F45DC7 126 GetCurrentObject
So I agree, the most direct route would be to fix the import library for GDI32.dll.
I still don't understand why GdiTransparentBlt is not documented in the 2003 PSDK. What reason could Microsoft have for not documenting it?