@andrewilley. Thank you very much for helping me with this.
I finally figured out what was the cause of my problem, and I already solved it with the help of foobar2000.
I am using Poweramp v3 build 945 (32 bits)
In my case, when dealing with mp3 files, Poweramp recognizes ReplayGain tags only if the keys are written in lowercase letters, which contradicts what you stated in his first comment.
My problem was because I was writing the keys of the RG tags in uppercase letters.
Another quirk (obviously a bug) that I noticed when PA parses RG tags in mp3 files, is that PA will completely ignore all RG tags if there are any duplicates similar to the following:
TXXX: (REPLAYGAIN_TRACK_GAIN) -5.02 dB
TXXX: (replaygain_track_gain) -5.02 dB
However, the following duplicate is allowed by PA:
TXXX: (replaygain_track_gain) -5.02 dB
TXXX: (REPLAYGAIN_TRACK_GAIN) -5.02 dB
Elias Villalobos's post in ReplayGain tag information inside mp3 files was marked as the answer