The above attached document may be useful in interpreting the meaning of the columns in this example FITS file.
Use scantable to import into an ASAP table (I think sdsave will also do that if outform is "ASAP").
If "sdsave" is used to export from an ASAP and outform is "SDFITS", the resulting FITS file will look this example.
Calibrated GBT data in an SDFITS format written by GBT tools (sdfits, gbtpipeline) can be read into a MS.
It requires additional tweaking of the MS to be useful for imaging.
The most significant work-around is that the MS does not support multi-feed data.
This workaround involves assigning the data from each GBT feed to a unique range of times/dates and artificially offsetting the times of the feeds so that there is no overlap in the resulting combined MS used for imaging. So long as those times are not used except as indexes into the various subtables (e.g. not used to recalculated az/el, or the line of sight frame velocity to calculate an alternate spectral window) then this hack works.
This has only been tested for spectral line GBT data. Continuum GBT data may behave differently if read in to a CASA MS.