«لِأَنَّ الْغَرِيبَ وَالْفَرْدَ مُتَرَادِفَانِ لُغَةً وَاصْطِلَاحًا»
Gharib and fard are closely synonymous in linguistic and technical usage.
Both words point to a kind of solitary transmission, though hadith specialists do not use them with exactly the same frequency in every context. It is inaccurate to treat them as wholly unrelated classes, but also unwise to assume their distribution in practice is perfectly identical.
Fard is more frequently applied to absolute singularity.
Gharib is more frequently applied to relative singularity.
This is a tendency of usage, not a rigid logical law that rules out the other application. The intended sense must be established through the actual context of the isnad discussion.