Michael:

The grid of time-gcm is:
Longitude: -180 to +180 by 5 degrees 
  (nlon=73, including one periodic point)
Latitude: -87.5 to +87.5 by 5 degrees (nlat=36)
Pressure (zp): ln(p0/p) -17., to +5., by 0.5.
  (nzp=45)

Mb pressures at each level = p0*(e^-zp)*1.e-3,
where p0 is standard pressure = 5.e-4, and
zp is the ln(p0/p) level. So, from bottom to
top:

k=  1 zp=  -17.00 pmb=  0.1208E+02 (~30km)
k=  2 zp=  -16.50 pmb=  0.7325E+01
k=  3 zp=  -16.00 pmb=  0.4443E+01
k=  4 zp=  -15.50 pmb=  0.2695E+01
k=  5 zp=  -15.00 pmb=  0.1635E+01
k=  6 zp=  -14.50 pmb=  0.9914E+00
k=  7 zp=  -14.00 pmb=  0.6013E+00
k=  8 zp=  -13.50 pmb=  0.3647E+00
k=  9 zp=  -13.00 pmb=  0.2212E+00
k= 10 zp=  -12.50 pmb=  0.1342E+00
k= 11 zp=  -12.00 pmb=  0.8138E-01
k= 12 zp=  -11.50 pmb=  0.4936E-01
k= 13 zp=  -11.00 pmb=  0.2994E-01
k= 14 zp=  -10.50 pmb=  0.1816E-01
k= 15 zp=  -10.00 pmb=  0.1101E-01
k= 16 zp=   -9.50 pmb=  0.6680E-02
k= 17 zp=   -9.00 pmb=  0.4052E-02
k= 18 zp=   -8.50 pmb=  0.2457E-02
k= 19 zp=   -8.00 pmb=  0.1490E-02
k= 20 zp=   -7.50 pmb=  0.9040E-03
k= 21 zp=   -7.00 pmb=  0.5483E-03
k= 22 zp=   -6.50 pmb=  0.3326E-03
k= 23 zp=   -6.00 pmb=  0.2017E-03
k= 24 zp=   -5.50 pmb=  0.1223E-03
k= 25 zp=   -5.00 pmb=  0.7421E-04
k= 26 zp=   -4.50 pmb=  0.4501E-04
k= 27 zp=   -4.00 pmb=  0.2730E-04
k= 28 zp=   -3.50 pmb=  0.1656E-04
k= 29 zp=   -3.00 pmb=  0.1004E-04
k= 30 zp=   -2.50 pmb=  0.6091E-05
k= 31 zp=   -2.00 pmb=  0.3695E-05
k= 32 zp=   -1.50 pmb=  0.2241E-05
k= 33 zp=   -1.00 pmb=  0.1359E-05
k= 34 zp=   -0.50 pmb=  0.8244E-06
k= 35 zp=    0.00 pmb=  0.5000E-06
k= 36 zp=    0.50 pmb=  0.3033E-06
k= 37 zp=    1.00 pmb=  0.1839E-06
k= 38 zp=    1.50 pmb=  0.1116E-06
k= 39 zp=    2.00 pmb=  0.6767E-07
k= 40 zp=    2.50 pmb=  0.4104E-07
k= 41 zp=    3.00 pmb=  0.2489E-07
k= 42 zp=    3.50 pmb=  0.1510E-07
k= 43 zp=    4.00 pmb=  0.9158E-08
k= 44 zp=    4.50 pmb=  0.5554E-08
k= 45 zp=    5.00 pmb=  0.3369E-08