Is it safe to take day quill while on 10mg celexa (citalopram) 100mg gabapentin and 150 mg of ranitidine?

Yes, but ...: I would recommend treating cold symptoms more conservatively. The active ingredients of DayQuil are acetaminophen (Tylenol), dextromethorphan (a cough suppressant) and phenylephrine (a decongestant). If anxiety is part of the reason you take Celexa and gabapentin, phenylephrine could make you more anxious. You would do better to use a nasal spray like Afrin to avoid the systemic effects.