The GX models come with glass heads, that Akai rated at 150,000 hours, as compared to 3000-4000 hours for a typical steel tape head. The first generation of GX heads came out around 1972, and continued until Akai stopped making reel to reel decks around 1985 or so. While in general the glass heads offer excellent performance, and do last far longer than steel heads, the glass can chip on the surface, or my theory is as well that they can crack internally. While it doesn’t happen often, I have seen the high frequency response drop off dramatically (sometimes around 7Khz) when a glass head gets chipped or cracked. These chips are difficult to see, as they are tiny, and I seem to see it more on the older, first generation Akai glass heads from 1972-1975 or so. Again, glass heads are unique to the Akai line, and while some models share the same head type, finding aftermarket used ones is difficult, and may take months.
Motor driver transistors- A number of GX models use a single transistor that is mounted on a heat sink inside of the deck. If this transistor fails, the capstan motor will be dead.
Bad relays – Some Akai models used relays to change the head selection in the audio path on some auto reverse decks. At 40+ years old, these relays can become intermittent, causing intermittent audio, or a noisy channel.