I think that you should give more information about your aim. You have both negative and positive charged compound in a single file, or you need to optimize the structures separately. After all, there is a section in gaussian input file, before coordinates section, that charge and multiplicity is defined there. The determined charge in this section is the total charge of the whole structure, but it can be defined for each component separately in spacial cases. It is why I recommend you to explain more about your work.
Simply, do the job with energy or in the case of optimization do optimization. Charged bodies have no concern with gaussian. Go to gauss-view and Cntrl+G and in the job section, select optimization or energy set for optimization or energy job running.
In fact, you could not assign charge to a specific atom in Gaussian input file and Gaussian needs the charge of the whole system as input. You just need to create your correct structure (taking from different chemical structure databases) and put the correct charge and multiplicity related to the whole structure in the Gaussian input file.