If I choose gathering for free development day will I still be attacked by other states / alliances..

Yes during a normal kill event weekend the other state can attack you on Sunday as well as Saturday and even if not then you can still be attacked if you have enemies in your state.

You can still gather though and if you do I recommend sending your APCs to different locations away from your base and separate from each other. This way if an enemy does find one of your gatherers they will not immediately see your other ones. With luck if you do get attacked it will only be one APC that is hit.