As in C, there are two ways to reference a file in Gamma, using a descriptor or a pointer.
A file descriptor is an integer that identifies an open file within a process. This is the lowest-level handle available for interacting with the open file. Disk files, pseudo-ttys, IP sockets, UNIX-domain sockets, pipes and other facilities all offer interaction through file descriptors.
A file pointer is an abstraction of the file that adds buffering on input and output. This would be of type FILE* in C. The reason this exists is that it is very inefficient to use a file descriptor, which does not perform any in-process buffering where many reads and writes are being performed. The file pointer stores many write requests until it has enough data to perform a more efficient write to disk, hopefully in multiples of the disk block size.
In Gamma, a file pointer is an opaque structure (the internals are not visible to the programmer) that is effectively a buffered FILE*. (See the note in open.) It's abstracted a little further to also include strings as file pointers, when they are opened using open_string.
Some Gamma I/O functions work with file descriptors (generally those that start with fd_), others work with file pointers, and a few work with both.