The GST/HST Rebate only applies to "employed persons" who are claiming Employment Expenses against their T4 income.
If you are Self-Employed and a GST/HST Registrant, then your GST/HST claim is totally separate form your tax return. GST/HST Registrants report the GST/HST collected on income, and any offsetting GST/HST paid on expenses as Input Tax Credits on a separate GST/HST Return.
More information can be found in this CRA link: CRA - GST/HST