spot means in fact another greyscale print. The printer can of course use any colour that is needed, and mostly it will be from the Pantone range.
This is only useful to add colours that fall outside the CMYK gamma, and is often used for fiiling jobs. It cannot be used as an extra channel for CMYK. Spot colours can never be judged on the monitor or your printer: only the Pantone book itself can help here.
Dot gain is the compensation that is given for the fact that ink penetrates the paper and slightly grows as it is absorbed. So the black dots become slightly larger, and the small white openings in the dark greys become also smaller. As Pierre said, contact your offset printer for this. It has no influence on your home printwork. Only for offset.
Grey gamma 2.2 means simply that the 256 greys of the greyscale are adjusted for a PC display.
Your monitor always displays Red, Green and Blue. When the three values are equal, you get greys. So when you calibrate your monitor, you do this in RGB, not in greyscale, even if you *see* the greyscale ramp.
When, in Photoshop, you set to Mode>Greyscale, PS offers you the option to see these greys as on PC or as on Mac. This has nothing to do with the monitor calibration or gamma-setting itself, in fact: it is an inverse compensation. So you may want to leave it as it is.
All this, and much more can be found in Inside PS7 by Gary (Gare from the Inside Track forum) Bouton.