Can you increase a yaw value from 0 to 1. For instance a gladiator at speed 3 wants to spend the dial and be allowed to click the first movement position by 1

Yup. From - to I or from I to II.

From the Learn to Play booklet, Commands section:

" Navigate : [...] Also, it may adjust one joint by one click more than the speed chart normally allows."

If the chart entry is "-" you would normally be allowed 0 clicks, so a Nav dial allows you to increase that to 1.

I don't think a different interpretation is possible, but I agree it could be a bit more explicitly worded.

Yep. DA did his thing.

Note that you can both increase yaw by one tick, AND you can change your speed. Those VSDs almost seem spry with an unexpected maneuver dial jumping to speed 2 and getting the extra yaw.

Yep. DA did his thing.

Quoters gonna quote, dawg.

Yep. DA did his thing.

Quoters gonna quote, dawg.

That's how we roll, yo.

Ok I like that ruling we always played if the speed had a "-" for yaw u were not allowed

In casual play you are certainly more than welcome to play with any house rules that your group mutually agrees on.

It punishes VSDs (and thereby the Empire) ridiculously more harshly than the rebels, though.

See also: Rules Reference pg 13, relevant part bolded.

Yaw

A yaw value indicates the maximum number of clicks from the center position that the corresponding joint on the maneuver tool can be clicked. A yaw value of “-” means that the joint must remain straight.

• Yaw values vary by ship and speed as displayed on a ship card’s speed chart.

• When a yaw value of “-” increases by one, it becomes “I.” When a yaw value of “I” increases by one, it becomes “II.” The yaw value “II” cannot be increased.

Oooh, I missed that. Thanks Kharnvor.

You're welcome.

It's literally the last thing in the glossary section of the rules reference. Very easy to miss.