I also trying integrate GC420T to android using OTG USB (not wifi or bluetooth).. But as rwest said in another person similar question (in attachmment):
"
We do have an Android SDK that does support USB connections using an “On The Go” cable.  It should work with the GC420. Some of the advanced API’s will not work as this is not  a Link-OS printer, but the basic communications API’s will.
Robin
"
 
 
Wich android SDK you talk ? Could you let me know version or link to download? I just want basic print with QR code.. Do you have any tips that could help conect it to android device? I am developing event ticket print solution for android.. and that is so much important for me to solve.. I depend of that to develop my startup company..
 
Thank you so much!
 
Best Regards,
Felipe