Gamma, soft clip and contrast only affect the LDR image, not the EXR. If you're working on an sRGB monitor you should leave the gamma at 2.2. This is equivalent to setting your "viewer" to sRGB (approximately) and at the same time outputting LDR images in the sRGB colour space (again this is only an approximation to sRGB).
Once you've done that, you're viewing something fairly close to a linear image. However, soft clip and contrast are both tone mapping effects that change the LDR image but not the EXR, so they'll cause surprises if you load the EXR into another app without reproducing these effects there. So you might prefer to turn them off in Terragen.
Matt