gems are just compressed tars, but are not seen as this:

[   55s] /usr/lib64/ruby/gems/2.1.0/cache/gem2rpm-0.10.1.gem differs (POSIX tar
archive)
[   55s] --- /tmp/tmp.BdC2jSeW8g    2015-10-27 23:42:10.512000000 +0000
[   55s] +++ /tmp/tmp.I5WDp2epjZ    2015-10-27 23:42:10.520000000 +0000
[   55s] @@ -3,8 +3,8 @@
[   55s]  *
[   55s]  00000060  00 00 00 00 30 30 30 30  34 34 34 00 30 30 30 30 
|....0000444.0000|
[   55s]  00000070  30 30 30 00 30 30 30 30  30 30 30 00 30 30 30 30 
|000.0000000.0000|
[   55s] -00000080  30 30 30 31 32 32 35 00  31 32 34 36 34 32 31 34 
|0001225.12464214|
[   55s] -00000090  30 30 37 00 30 31 33 34  33 36 00 20 30 00 00 00 
|007.013436. 0...|
[   55s] +00000080  30 30 30 31 32 32 35 00  31 32 36 31 34 30 30 35 
|0001225.12614005|
[   55s] +00000090  37 31 36 00 30 31 33 34  34 30 00 20 30 00 00 00 
|716.013440. 0...|
[   55s]  000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 
|................|
...