Atm the TRM isn't letting me download it from the Cypress site, but I'll try again later. At a guess, it would be somewhere on the order of 1.5 dB, 3 dB, or 6 dB (3 dB is double).
Looking in the TRM, page 347 refers to the LNA gain setting of 101 being 66 dB, and the potential to boost the sensitivity by 2-3 dB at the expense of increased current at that setting level (sounds like it will boost it at other levels with little-to-no effect.)
With the highest being 66 dB, and the lowest being 0 dB, I would see the steps as 13.2 dB
I saw the component document at the link below.
In p84, I saw the function "CyBle_SetRxGainMode"
In the function description, the "CYBLE_BLESS_HIGH_GAIN_MODE" setting is called -91dB.
What is the difference between minus db and plus db?
And how does the LNA Gain and CyBle_SetRxGainMode settings match?
Positive dB is a stronger signal; Negative dB is the same proportional change in signal strength, but it is decreased signal strength instead.
The Normal vs High Gain modes differ by the 2dB difference, which is a roughly 1.6~ times increase in signal reception sensitivity.
The radio lists -89 dB as the default RX sensitivity as that is the fractional power of a signal from a unity-gain transmitter that can be correctly received. For example, a radio transmitting at 1 W/m^3 would be receivable by the receiver at a -89 dB drop in signal strength before reaching the receiver, this dB value corresponds to 0.0000000126 W/m^3 of signal strength needed to function. (This is a rough calculation, but it demonstrates the math and concept pretty well.)
No idea how the LNA Gain matches to the CyBle_SetRxGainMode() function, but it could be a difference between the TRM and the cypress function that sets the settings found in the TRM to specific configurations for each setting/mode.