Necessity of Gully Trap?

Not sure of your situation but the floor trap in my house don't link up to the sewage. Where gully traps or U bends (S bends) do, hence the need for them.

Floor traps generally have P traps which wont allow foul gases to enter houses. This waste line can be connected to the sewage through some inspection chambers in between( 1 or 2 depending on the distance). If that is the case , then in what way the gullt traps help?

If the water has evaporated from your trap, gasses can come in.

If that is the case, are there not any chances of water in gully trap evaporating?

Yes, they can evaporate. It's hard to tell your situation, but just a guess, if they do evaporate and you are getting smells inside the house, just add enough water periodically to keep the trap full.

To comply with local building regulations!

One purpose of a gully trap is to provide a low level exit of waste water if in fact the sanitary line backs up. It is placed outside the home so the backed up over flow exits there instead of from the drains on the interior of the home.