Actually film can have a gamma of 1. This is a measure of the straight line angle and then finding it's tan (trig). The tan of a 45 degree straight line is 1. When making paper, we wish to match the contrast of the film to the contrast of the paper. It was discovered that making the film contrast bases on a straight line angle of about 38 degrees = gamma 0.80 is a best fit for a grade 2 paper. A lower gamma has too little contrast and an higher gamma has too much contrast. This is a film to paper fit thing. It is based also on the fact that we view prints by reflected light that passes through the developed emulsion and then returns through the same emulsion. In other words light makes two passes. Because of this, one stop change of exposure on paper is double the blacking that would occur if you increased the exposure to film. 1 stop plus on film what a gamma of 1 = 0.30 density units. With a gamma of 0.8, one stop change is 0.8 X 0.30 = 0.24 density units. Paper with a gamma of 2 yield 0.60 density delta for 1 stop of exposure increase. Again, paper reacts twice as fast as film due to the two light passes and the gamma of both film and paper are adjusted to make a best fit as to contrast.
The ASA method of testing for film speed and now the ISO method is based on 0.10 above base fog. The film is sluggish as this is about the threshold of exposure. Best to start measuring just above this toe area. After all, the straight line area is due to a uniform density change induced by uniform changes in exposure.
2.00 is about the maximum blacking you can get on film. X-ray film is coated both sides and goes up to 4.00. The density of 2.00 is a log value base 10. It is 100 in decimal notation. A 100 watt lamp appears as if it is only 1 watt when viewed a film with a density of 2.00. Some pictorial films go to Dmax at 2.50 density.