The problem is the space in your assignment. The shell is whitespace-sensitive. Your command is the equivalent of:
var="" "./gen.4.bt2 ./gen.rev.1.bt2 ./gen.rev.2.bt2 ./gen.1.bt2 ./gen.3.bt2 ./gen.2.bt2" # output of find
You need to remove the space:
var="$(find . -name 'gen*.bt2')"