No they are not the same. Shotgun means random fragmentation and sequencing, in theory, all bases from your genome will be covered given enough sequencing. RADseq means restriction-site-associated DNA sequencing, and only regions surrounding restriction enzyme sites will be sequenced.
GBS may mean the original restriction enzyme associated protocol from Cornell (I think some people refer to it as Cornell GBS), or a general term which comprises several techniques. Indeed, skimGBS is a shotgun genotype by sequencing method.
See the references cited here and here for details.