A short, vertical antenna suffers from several disadvantages, low base resistance, high values of reactance and excessive high-angle radiation. top-loading in its several forms is not only an attempt to contribute to the electrical length in an effort to more closely approximate a full 1/4 wavelength but to also mitigate both radiation resistance, input impedance and current distribution, to balance those values to produce more efficient radiation of the em wave and prevent excessive I2R losses wasting transmitter energy in the form of heat. the gain figure associated with the GR45 @ 4.75 Dbi is more believable than most, and in case some of you missed it IS compared to the "unity" gain produced by the isotropic source, Db = decibels, i = isotropic. as 351 alluded to in his post, this antenna performs at its best when used in environments such as hilltop vantage point terrain and also in wide-open unobstructed areas that are mostly rural in nature as the top-hat also serves to eliminate large amounts of wasted upward radiation only useful in more suburban and urban environments or for high angle, short hop sky wave propagation. with the GR45 operating in the type of terrain that it was designed for, you will be hard pressed to find anything else in a 4'3" package handling 12KW of power that will perform as well. and with one last reference to the quoted gain, 4.75 Dbi, 2.64 Dbd, try constructing a 1/2 wave dipole at the same feedpoint height (if you can) as a comparable installation of a GR45 and see for yourself how quickly the ground losses at that height above ground just eats up any gain that might have been produced by the dipole because of its close proximity to ground and you'll get a glimpse of just exactly what some of the other functional attrubutes of capacitive top-hat loading actually contribute to an antenna design such as this.
so there are at least 5 quantitative values that top-loading attempts to balance and affect in a favorable manner, radiation resistance, impedance, I2R/ground losses, electrical length and elimination of wasted skywave radiation in an attempt to increase the efficiency and the gain of a physically shortened 1/4 wave radiator. now it's no longer a mystery.
