Can someone please explain to me the difference between these two switches GS305 and GS105NA?

> [...] I looked the specs [...]

   Which "the specs"?  Visit http://netgear.com/support , put in your
model number(s), and look for Documentation.  Get the Product Data
Sheet(s).

   I don't see any significant differences.  Length of warranty?

> Is the difference that the GS105NA can connect up to 2gibit per port
> and the GS305 only makes up to 1gbit?

   No.  Both have 1Gb/s ports.

   There's a more appropriate forum for such devices:

      https://community.netgear.com/t5/x/bd-p/business-unmanaged-switches