These are the following basic-dialects which are developed:
gambas
freebasic (advanced improvement over qbasic)
scriptbasic
chipmunkbasic
yabasic3
thinbasic (powerbasic syntax).
All these can also be used for making gui applications which differ with each other, and also webserver pages under cgi script mode.
If you are very particular with gwbasic compatibility then choose bwbasic (gwbasic enhanced for freebsd and linux).
However for graphics you have to contend with x11 based options or wait until it adds support for tk and opencv.
If you are willing to look for:
robust vb-like development environment + basic server pages, then choose gambas, otherwise there is a new one coming known as fbsl (freestyle basic scripting language) based on freebasic-syntax.
robust web-engine for basic server pages, then scriptbasic, and thinbasic
syntax influenced with python/perl/ruby, then yabasic3
easy and very standard syntax: freebasic, otherwise even gambas and yabasic3 are very easy to learn.