You can tupletize lone notes with no warnings:
### BEGIN ###
\new Staff
{
\times 2/3
{
g'2.:8
}
\times 2/3
{
a'2.:8
}
\times 2/3
{
bf'2.:8
}
\times 2/3
{
bf'2.:8
}
}
### END ###
Changing "\tuplet 2/3 2" to "\tuplet 2/3 4" (David's solution) is the right one; but knowing about the (explicit) time scaling available with \times is helpful to have as a backup (and allows you to think and notate each sixth note as a sixth note).
Trevor.