$ usermod -a -G dialout andreas

$ strace gtkterm
[...]
stat("/var/lock/lockdev", {st_mode=S_IFDIR|0775, st_size=40, ...}) = 0
open("/var/lock/lockdev/LCK..ttyUSB0", O_RDONLY) = -1 ENOENT (No such file or directory)
umask(022)                              = 02
open("/var/lock/lockdev/LCK..ttyUSB0", O_WRONLY|O_CREAT|O_EXCL, 0666) = -1 EACCES (Permission denied)
write(2, "Cannot create lockfile: Permissi"..., 42Cannot create lockfile: Permission denied) = 42
[...]

$ usermod -a -G lock andreas