die Gefährtin
companion
A female person who accompanies someone on a path, during an undertaking, or through a stage of life.
Sie war ihm auf all seinen Reisen eine treue Gefährtin.
She was a loyal companion to him on all his journeys.

companion / partner
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companion
A female person who accompanies someone on a path, during an undertaking, or through a stage of life.
Sie war ihm auf all seinen Reisen eine treue Gefährtin.
She was a loyal companion to him on all his journeys.

The word belongs to the word family of 'fahren' (to drive/travel). A 'Gefährtin' (female companion) was originally someone who shares a 'Fahrt' (journey/trip) or a path with you. This idea of a shared path is present in both meanings.
Sie war seine Gefährtin auf der langen Wanderung durch die Alpen.
She was his companion on the long hike through the Alps.
'Gefährtin' (female companion/partner) is a rather elevated and sometimes slightly poetic word. It expresses a deep and lasting connection.
In seiner Rede dankte er seiner Gefährtin für die jahrelange Unterstützung.
In his speech, he thanked his partner for years of support.
When a companion for a specific activity is meant, this is often made clear by a preposition, e.g. 'Gefährtin auf seinen Reisen' (companion on his travels) or 'Gefährtin im Abenteuer' (companion in adventure).
Die Hündin war dem alten Mann eine treue Gefährtin bei seinen Spaziergängen.
The dog was a loyal companion to the old man on his walks.
In the meaning 'life partner', the adjective 'treu' (loyal/faithful) is often used ('eine treue Gefährtin' — a loyal partner). A very common and clear form is also the compound word 'Lebensgefährtin' (female life partner).
Er stellte sie allen als seine Lebensgefährtin vor.
He introduced her to everyone as his life partner.
girlfriend / female friend
'die Freundin' and 'die Gefährtin' can both refer to a female person who is close to someone. 'die Freundin' is much more common in everyday language and can have two meanings: a good female friend or a girlfriend in a romantic relationship. The context often shows what is meant. 'die Gefährtin' sounds more elevated or somewhat more formal. It means either a companion on a path or in a situation, or a committed, usually long-term life partner. The words can be confused because both can express a close relationship. In normal conversations, people usually say 'Freundin'. 'Gefährtin' fits better when you want to emphasize companionship or a lasting partnership.

Auf der langen Reise war seine Gefährtin immer an seiner Seite.
On the long journey, his companion was always by his side.
Nele stellt ihrer Familie heute Abend ihre Freundin vor.
Nele is introducing her girlfriend to her family this evening.