Constant Gain-Bandwidth product in amplifiers
can somebody explain to me how the gain and bandwidth product is
constant in an amplifier or OP AMP ?

Just look at the open loop gain of a typical op-amp: -

Mathematically this happens because an op-amp internally is equivalent to a DC gain stage with high gain followed by a single order low pass filter and, as we know with a single order low pass filter, the amplitude reduces above the 3 dB point at a rate proportional to frequency i.e. ten times the frequency means one-tenth the amplitude.
It's all embodied in a simple RC low-pass filter when we move beyond the cut-off frequency.

An RC low pass filter has a transfer function of: -
$$\dfrac{1}{1+j\omega RC}$$
And, when you get beyond the cut-off frequency the transfer function becomes asymptotic with \$\dfrac{1}{j\omega RC}\$ hence, gain is inversely proportional to frequency.
That slope has a constant gain-bandwidth product.