There is no conflict. Gita is talking about the Atman. Garuda Purana is talking about the linga sarira (also known as sukshma sarira) or the subtle body.
The human body and personality are contained in 5 sheaths according to Vedanta:
- Annamaya kosha
- Pranamaya Kosha
- Manomaya Kosha
- Vignanamaya Kosha
- Anandamaya Kosha
Of these koshas, Annamaya Kosha is the gross body. The next three koshas make up the subtle body or linga sarira. The last Anandamaya Kosha is the causal body or karana sarira.
There are five kosas or sheaths in which the Self is manifested as the
ego or the jivatman. The first of them consists of food. Other
sheaths consists of prana or life, manas or instinctive and
perceptual consciousness, vijnana or intelligence and ananda
or bliss. These five principles of matter, life, consciousness,
thought and bliss are found in the world of non-ego. Anna or food is
the radiant, the viraj, which is perceptible by the senses, the
physical. According to Suresvara, life, consciousness and intelligence
constitute the subtle self, the sutratman and bliss is the causal
sheath, the karana kosa.
Commentary on Taittiriya Upanishad II.1.1 by S. Radhakrishnan in The Principal Upanishads
After these 5 koshas lie the Atman. The mind is a small part of the subtle body. Even ghosts have subtle and causal bodies.