Gastric tubes are inserted through the mouth, and nasogastric tubes are inserted through the nose.
Gastric tubes are usually large in diameter, so large that they cannot fit through the nostril of the human nose, and nasogastric tubes are small in diameter and are easy to pass through the nostril (through the nose). Some nasogastric tubes are specially coated and weighted to facilitate passage through the nose. Gastric tubes are not specially coated.
A gastric tube (aka nasogastric) can be inserted through the mouth and nose. Depending on the situation. But it is better to do this through the nose - there is no danger that the patient will chew the probe and significantly less pressure on the root of the tongue, which reduces the gag reflex.
The probe, which is supposed to be placed for a long time, can only be installed through the nose .
For quick gastric lavage, often with a narrowed nasal passage, it is permissible to put through the mouth.
Gastric tubes are inserted through both the mouth and nose. The problem with the mouth is that there are teeth in it and this probe can be eaten. To avoid such problems, the probe is inserted through the nose.
I'll correct you a little, these probes are called nasogastric. And they are inserted through the nose for the reason that when introduced through the mouth, the path of the probe will pass through the root of the tongue and the back wall of the pharynx, causing the gag reflex.