It appears as though SGP has a bug with reporting the egain of a ZWO camera when using the built-in SGP “ZWO Camera” driver. I did some testing, and it appears that when you adjust the gain setting, which is a simple integer such as 76, 139, 200, etc. that the gain does indeed change. Comparing subs of different gains but the same exposure shows definite differences in line with the gain setting. The gain FITS header is also reported properly.
However, all of these test images end up with the same exact egain FITS header. The driver seems to report the minimum gain of the camera which it reads as 4.96… e-/ADU, regardless of the actual gain setting used. Gain 139 on an ASI1600, for example, is unity gain, so it should read ~1e-/ADU. Gain 76 is exactly half unity, so it should read ~2e-/ADU. Gain 200 is a little over double unity, so it should read ~0.496e-/ADU. All of these gain settings read the same egain value as Gain 0, which is 4.96e-/ADU.