When shifting into one particular gear grinding sounds occur.
Troubleshooting car problems of grinding noise when shifting
If the grinding sound happens only when shifting into forward gear the usual cost of the problem is the synchronizer of the affected gear. Synchronizer a transmission components that brings the rotating shafts/gears together at the same speed so that the shifting will become smooth. Now, if the synchronizer is defective you can hear grinding sound every time you shift into forward gear. If the defect on the synchronizer is severe then shifting to another gear will be impossible.
However if the shifting problem only occurs when shifting from a stop into first gear, it is possible that the cause is the clutch pedal free play. Clutch pedal free play is the movement of the clutch pedal before the clutch starts to disengage. When there is too much clutch pedal free play even when the pedal is fully depressed the clutch will not be fully disengage when shifting to first gear in this case the grinding sound is heard when shifting.
Free car diagnostic recommendation on grinding noise when shifting
This kind of problem can only be fix by a mechanic that specializes when transmission repair. With that said, no other option but to bring the car to a shop specializes for transmission repair. When your problem happens only during shifting from stop into first gear then the mechanic might only need to adjust the clutch pedal free play. If the mechanic recommend opening the transmission never agree, seek first for other shops opinion before agreeing to the transmission overhaul.