I find the solution as:
string.encode "IBM437"
as said in comment, I reread the Epson TM-T20 printer; It's setted with default "code table" number: 0, meanining: "PC437: USA, Standard Europe"
In fact I understand PC437
refer to the 2Code Page 437', see:
en.wikipedia.org/wiki/Code_page_437
So i found the interesting gem: github.com/ConradIrwin/encoding-codepage
showing that Code page 437 correspond to charset "IBM437"
Now printer print correctly! I answer here myself to maybe help others.
giorgio