Compatibility of GigaRec get's worse the more you deviate from the 'standard' CD-R format (i.e. no GigaRec). From the possible settings 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3 and 1.4 I'd strongly recommend you go not lower/higher than 0.8/1.2, unless you have a very good reason for doing so. To illustrate:
Gigarec @1.3 with more errors that a normal burn
Gigarec @1.4 with loads of errors
CD players always read @1x which is less demanding than higher speed. Reading GigaRec/AMQR means the standard read speed 1.2m/s or 1.4m/s @1x needs to be adjusted. Something that is not trivial, because when going beyond a certain point you'll move outside possible speeds set in the red book standard or possible tolerances of that standard.
Same as on normal discs. The predefined limits by the standard however (220 max. for BLER for example) need to be seen with a grain of salt, since the disc was (eventually) created to be outside of the standard to begin with.
See my other question: What CD-R did you use? Some type of media don't like GigaRec/AMQR. Look here where AMQR gets off worst (albeit still ok) compared to 40x and 2x burns:
Other than that: Red Book (definition of Audio CD) defines two speeds for CDs:
63min with 1.4m/s
74min with 1.2m/s
(80min as 74min with the pre-groove spiral of the data track been narrowed)
Basically this means the extra minutes of the 74min CD are created by turning the (same) CD slower (the same amount of data passes the laser but needs less space). AMQR (=GigaRec at a rate of 0.85) simply reverses that trick. Turning a 74min CD @1.4m/ makes a longer area pass the laser in the same time (the same data takes up more space). Since AMQR thus stays within Red Book speed definitions it should, in theory, be readable by any drive. However practical experience shows that drive manufacturers seem to have a different view of things (possibly assuming that a 74min or 80min disc is always to be read at 1.2m/s or multiplies thereof. But that's a wild guess. Nobody can tell for sure. It would mean a deep look into the firmware.) Computer drives seem particularly more affected by this than audio drives in stereos.