At first i had a problem understanding dbi and db. After much reading and trials, i finally understood what it meant.
Actually, when you do a simulation in HFSS, you won't see a dbi term. But rather you see a dB term. What it exactly meant is that HFSS actually compared the gain to an isotropic antenna. For your info, isotropic antenna is actually an ideal antenna which emits gain of 1 (absolute value) equally in a sphere form. So the gain in dB of an isotropic antenna is 10log(1) = 0dB.
So the dB you get from your HFSS simulation is actually the real value of the antenna gain and it's auto compared to a isotropic antenna so you can assume it's equivalent to a dBi form.
The gain of antenna is not actually the gain of the antenna itself, but its always a comparison of gain towards another antenna. So once you get your gain in HFSS simulation results in dB result, you can equate it in dBi. So when you do a real time measurement in an anechoid chamber, you realise that there's no such thing as an ideal antenna. So you will have a transmissiting antenna (could be anything) sending signals to your fabricated antenna (receiver).
When the measurement ends, you can see the network analyzer will show your gain in dBi format which actually meant that your antenna in comparison to the receiver is how much less or how much more. dBi doesn't mean the gain value entirely, it's just the difference of gains between 2 antenna.