What GC and what software? I have a 6890 in my lab and you can set up manual runs just using the keypad on the GC itself. 

Yes, possible

Believe it or not, gas chromatographs were invented long before autoinjectors, and for years chromatographers did manual injections :-). I still do nearly all my injections manually.

Change the settings in the software, load a syringe with the required injection volume, smoothly insert the needle through the septum, count 2,3, depress the plunger, count 2,3 and remove the syringe. No human can mimic the fast-cold-needle injections of the Agilent autoinjector, doing it with the 2s pauses gives repeatable manual injections. 

Thank you all

Yes. It is possible. Manual injection of the liquid or gas samples with using the glass syringe is very popular and used more frequently than injection with application of the autosampler. After injection of the sample into the injector You must very quickly push button START. 

To load the syringe properly, either invert the septum filled vial and put empty syringe needle  into the liquid or vapor whichever you wish to inject into the inlet.  Then pump the syringe a few times to remove air.  Then pull up your sample  to the rignt volume you wish to inject.  Pull the syringe out of septum and inject  into the inlet with a single thrust.  DO NOT PUMP THE SYRINGE.

For manual injection, I always taught cleaning and pump-wetting a micro-syringe with superclean n-hexane, filling the needle and syringe cavity before ejecting excess and pulling up the sample. A small but measureable amount of this n-hexane is visible above the sample with a bit of air, which is then pulled up into the syringe for inspection.  When this sample is injected, this remaining clean hexane plug will wash out the entire sample, thus including needle content.  (Turning a vial upside and washing its septum: I don't like).