Is das belegtes Brot not the german for sandwich or is this a loan word that is in common use ?
Sandwich indeed is a common loanword. They're often interchangeable. From my experience, belegte Brote are often less garnished and mostly without a second slice of bread, but that might just be me. dict.leo.org suggests 'open sandwich', too.
How can Sandwich be neutrum and maskulinum on the same time?
It's a loanword, it's new to the thesaurus, you have to choose between three articles and none of the actual rules apply. Which one do you pick? It's the same with blog. Both der and das are used.