The highest attainable frequency by an oscillator depends on the technology used, with semiconductor oscillators reaching around 200 GHz and multipliers potentially boosting this to several Terahertz. Photomixing can generate frequencies in the Terahertz range by mixing two lasers. While gas lasers can emit visible light, producing visible light from an antenna is not feasible due to the size constraints of antennas compared to the wavelengths of visible light. Electromagnetic waves at any frequency can be generated through various methods, but simple antennas cannot produce gamma rays due to their extremely short wavelengths. Overall, the discussion highlights the complexities of frequency generation across different technologies and the limitations imposed by physical properties.