Sometimes when I'm praying alone, I like to recite a surah out loud because it helps me concentrate, and because for some reason, it gives me more motivation to recite longer surah's. Am I allowed to do so? Or is it only permissible to recite a surah out loud when in a gama'a?
It is allowed. Many sahabah used to do it. Prophet Muhammad (SAW) used to listen to the recitation when he (SAW) used to walk in the streets at night.
One night when Prophet (SAW) was walking in front of the house of Abu Musa Ashaari (RA), he (SAW) heard him (RA) reciting during salah. He (SAW) stayed there for a little while to listen to this recitation. The following morning after Fajr, he (SAW) praised Abu Musa Ashaari (RA).
Narrated By Abu Musa:
That the Prophet said to him' "O Abu Musa! You have been given one of the musical wind-instruments of the family of David."
Sahih Bukhari Volume 006, Book 061, Hadith Number 568.
Abu Qatadah narrated that : The Prophet (peace be upon him) said to Abu Bakr: "I passed by you while you were reciting and your voice was low." He said: "I let He who, I was consulting hear." He said: "Raise your voice." Then he said to Umar: "I passed by while you were reciting and your voice was loud." So he said: "I repel drowsiness and keep Ash-Shaitan away." So he said: "Lower your voice."(Tirmidhi)