gretl is very good at figuring out what you intend to do. In this case, you intend to create a series and omitting the genr command does this, probably because you are creating a data series from another series. The problem is, sometimes this isn't what you want. Say you want to create a series from a matrix.
The genr command is a do-it-all command, but it suffers from the same problem associated with using no declaration. gretl may not know that you expect a series and genr will compute something based on whatever you are using on the right hand side of the assignment operator. I now use more specific commands like series, matrix, scalar, list so that I can keep track of exactly what I want to compute. genr will do all of these, but it really cannot tell which of the specific types you are intending to compute. genr does have a few special uses that the others don't, so there are times when genr has to be used.
For creating series, though, I prefer
series y = variable_name
to
genr y = variable_name
though both generate the same thing in most circumstances.
Lee