Often discussing ground penetrating radars or in advertisements of manufacturer they say "500 MHz georadar antenna." Also, this frequency is indicated directly in the name of the antenna "VIY5-300" or "200 HS Antenna". What frequency are we talking about?

There are two main types of ground penetrating radars - Video-pulse and Frequency-modulated continuous-wave (FMCW). Both of these types are ultra-wideband. It is not clear what this one frequency means when describing such devices.
For example, for detecting underground utilities, VIY5-300 georadars with 300 MHz antennas are often used.

In the picture, the spectrum of GPR signals is highlighted in blue, and the spectrum of signals from a radio station in red. Obviously, the blue line is difficult to characterize with a single number.

It is more correct to say that the signal has a spectrum maximum at a frequency of 300 MHz and at the same time indicate the bandwidth.? But most GPR manufacturers omit the bandwidth parameter for simplicity. Although, if you specify the width of the probe pulse (for VIY5-300 it is 3 ns), this parameter unambiguously describes both the center frequency and the width of the spectrum.

So, the frequency in the name of the antenna is the maximum value in a wide spectrum. And this does not mean that the ?200 HS Antenna? operates at a frequency of 200 MHz.