Modify a response file gain, in a particularly simple way. 
CAUTION This
command is to be used with extreme care for investigation of the response
properties. To properly fit data, the response matrix should be recalculated
explicitly (outside of XSPEC) using any modified gain information derived.
XSPEC> gain < <file no.> <slope> <intercept>...
The initial default file number is 1; later, the default is the number of the
file last modified. Initially, all responses are assumed to have nominal gains,
determined implicitly by the data in the response files. This is equivalent to
a <slope> of 1 and an <intercept> of zero.
The gain is automatically reset to nominal whenever a new data file or response file is defined for that file number, but not by any use of the ignore, notice, or dummyrsp commands.