[Ternas]

While realistically a cloak should probably provide DX penalties, especially if it grants a bit of DR, we can see from Low Tech pg, 99 that a cloak is a many varied item but by far the most basic one as described on Low Tech Instant Armor pg.20 that cloaks, by default, are only knee length with no hood and protects only torso and thighs from the back only.

Now how this actually affects you comes into play from Low Tech pg103 on layering armor where it only provides penalties where a roll greater than 1-3 out of 6 protects you and I'm going to have to assume, logically, that if you're only protected from the rear, you're not going to exceed that 1-3 roll.

Of course this would probably only apply to the GURPs basic cloak. As said there are myriad versions, including the poncho.

The poncho from Low Tech pg. 99 is similar to a cloak, but it goes over the arms and the front. As the version detailed there isn't one that provides actual DR, it merely interferes with your arms giving -1 DX. I would assume if you somehow managed to create a Heavy Poncho it would, like the Parka above it, grant a complete -1 DX penalty for layering armor on the torso which affects everything you do.