Mariusz did a great job !
But his explanation is somewhat short.
Unfortunately I can't access your data, but I would proceed as follows.
Looking at your data-points there seems to be something like a straight line at the right and left end. Get some of the first and some of the last data and do a linear fit, then you will get a first approximation of a linear function to be used as "baseline". Then extract the data in the middle. These could be a Gaussian like
a + b Exp[ w ( x - x0 )^2 ]
a is something around 100, b as well and w has something to do with the half-width, which can be estimated as about 0.02. So find a value for w. Fit it to the extracted data, and you will get starting parameters for the overall fit of all your data to
a + b Exp[ w ( x - x0 )^2 ] + c x
