Simple assault. Depending on the jurisdiction, maybe battery. The elements of battery are an intentional act done for the purpose of causing a harmful or offensive contact.
um,no. You are ignoring intent, which is the ESSENCE of The Law (tm).
If they are trying to remove your gun from your person and are using force against your resistance, it's attempted robbery.
Assault (or A&B depending on jurisdiction) would be a lesser included misdemeanor. The charge would be attempt robbery (to avoid the misdemeanor joinder rule upon arrest)
Generally speaking, attempting to remove property from a person via force is attempted robbery under most penal codes. Generally speaking, force must be used. Iow, if a person is carrying a handbag and a guy runs up from behind and takes the bag without having to overcome force and the person doesn't even know it's happening until it's removed from him, under some penal codes, that is "theft from person" or simple theft/larceny.
If the person knows it's happening and resists or the person uses force or threats to overcome resistance, it's robbery (or in the case presented here - attempted robbery)
So, it depends if the person tugging on it is overcoming your resistance (attempted robbery) or not. Also,. any assault committed in the commission of a theft will up it to robbery. We charge these all the time when shoplifters assault loss prevention officers - robbery.
If the person tries to disarm me, I know that over 80% of the time a person disarms a LEO, they use the gun against the LEO. It's a deadly force situation. We had a situation with one of our officers where an armed robber got her gun out of her holster and they were fighting over it. Store owner came up and shot the robber in the head. Good shoot.
My state has a specific statute for disarming that does not require the overcoming of force element in a robbery, and is a C felony. If they overcome force, or get the gun via threats, etc. then it is robbery.
Here's a good lesson on robbery
http://legal-dictionary.thefreedictionary.com/robbery