There must be a gem for thatSure enough, the barby gem. It allows you to output your barcode as png, gif, svg, pdf, etc...
In my case I generated png barcodes and included image references to those in my view files.
There is no pixel setting, it uses some other type of unit to determine size; using the xdim, margin and height options you can tweak the size, but I found that it was not very precise - for my use case it was good enough.
The generated barcode does not include the value, it is just the image which can be scanned using a barcode scanner, but it would be nice if there was an option to include the value below that. This is somewhat trivial to add in your view using html.
Overall I was really pleased with this gem!