this is a gold bar. If we look at its structure, will we see that there are atoms that have a different number of neutrons (aka isotopes) (as far as I'm concerned, an isotope is an atom with a different amount of neutrons in it) or has it to be another bar that consists of this isotopes?
Gold has only one natural isotope: 197. There are artificial gold isotopes, but they do not end up in gold bars. Even if they did, their half life is measured in days so don’t expect much to remain after a year or so.
The “structure” of the metal bar, or atomic arrangement, has nothing to do with its isotopic composition.
Even if you were to measure somehow trace amounts of radioactive gold in your bar (for example gold-195) using methods such as mass spectrometry, it would be hard to distinguish from platinum-195, a very common impurity in gold. To complicate it even more, you will not know whether the Pt-195 was there to begin with, or formed by the decay of Au-195.
If you had a gold bar made of pure gold-195, you would be dead soon from radiation sickness unless you took the appropriate safety measures.