Frank said:

For interfacing to C in GPC use the C compatible types (see manual). For fixed size types use `Integer attribute (Size = 32)' or `Cardinal attribute (Size = 32)' etc. I don't know what FPC has.

FPC 1.9.x+ has in the "ctypes" unit all C types prefixed with "c". So clong, culong, cuint32 etc.

Not all existing headers are converted to this already. Most older header conversions were submitted by users, so they use all kinds of conventions. Some dedicated users are slowly "ctypes"ifying them.