What does guchi mean?

Specifically Miyajimaguchi? Or Minamiguchi?

Most likely train station exits. Minamiguchi means South exit, Miyajimaguchi means Miyajima exit.

Kuchi (or guchi) means mouth, portal, or entrance/exit. In the case of Minamiguchi south exit/entrance would be appropriate and is indeed often seen at train stations.

Miyajimaguchi on the otherhand means more like the entrance/portal to Miyajima and is the name of the train station that sits at the connection point to Miyajima Island. Name+guchi or Name+mae (mae meaning in front of) are common names for train and bus stops near major sights.

[kuchi] becomes [guchi] in most compounds where it means gate, portal, entrance, exit or so and follows another word.

Miyajimaguchi --- [kuchi] to Miyajima, entrance / portal point (on passage leading) to Miyajima
Minami-guchi --- [kuchi] in [minami], gate / entrance / exit in the south
Kaisatsu-guchi --- [kuchi] for [kaisatsu], gate for inspecting tickets

__-guchi of train stations in Japan are not necessarily ticket gates.
e.g.
A traveler from overseas and his Japanese friend decided to meet at [Minami-guchi] of XYZ Sta.
He searched for [Minami-guchi] in XYZ Sta., but no sign in English was saying "South Gate" or "South Exit."
Feeling anxious, he asked a station attendant, showing his note: "Where is South Ticket Gate?"
The attendant replied, "That means the south entrance. Please pass through the gate there, turn to the right, walk a littlie and go downstairs."

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