Yes, it's fine for P3...as are all green veggies except peas, which are high in starch.

While P&I doesn't give us a specific list or guidelines, it can help to figure out how much (if any) sugar and starch is in a food by checking the carbohydrates. Carbohydrates measure three things: fiber, sugar and starch. You don't have to worry about fiber since it passes through undigested and has no calories. So you can see how much natural sugar and starch is in foods when taking that out. It's nearly impossible to avoid ALL sugar and starch unless you eat only meat and fat, so the goal is to focus on plant foods, nuts, and dairy with the lowest possible natural amounts. Spaghetti squash looks pretty low in both. Here are some comparisons, all for 1/2 cup servings

Data from calorieking.com

Zucchini squash, 1/2 cup sliced
Total Carbs 1.9 g 1 1%
Dietary Fiber 0.6g 5%
Sugars 1g

So when you remove the fiber out, you get 1.3 grams carbs left, 1 gram of which is natural sugar. So 0.3 is natural starch.

1/2 cup spaghetti squash
Total Carbs. 4.7g 2%
Dietary Fiber 1g 4%
Sugars 1.8g

So when you subtract the one gram fiber out, you get 3.7 grams left. Since natural sugar is 1.8, the rest is starch.

Here is a higher starch veggie, you can see how the carb count is much higher.

1/2 cup peas:
Total Carbs. 11.4g 4%
Dietary Fiber 4.4g 18%
Sugars 3.7g
So when you subtract fiber out of the carb count, you are left with 7 grams. Since 3.7 grams are natural sugar, the rest is starch.

Just to show nearly all foods have sugar and starch in small amounts, here is lettuce.

1/2 cup Romaine lettuce
Total Carbs. 0.8g 1%
Dietary Fiber 0.5g 4%
Sugars 0.3g

The glycemic index is a different concept. I am just comparing nutritional data. However, foods that are very low in carbohydrates are also low-glycemic foods, that means they won't raise your blood sugar much.

I know it can all be confusing, unfortunately Dr. Simeon didn't provide much guidance other than "no sugar no starch" in P3. He doesn't make any distinction between natural sugars (as in fruit or plain yogurt) and added processed sugars (like fructose corn syrup). He also doesn't provide a comprehensive food list or total carb count to consider. However, he does say that fat and starch are a particularly dangerous combination in P3, stating that one's weight can "quickly get out of hand" when these two are combined. So to many that means higher amounts of natural sugar, such as in fruit or dairy, is OK as long as there is no added sugar, but foods that are higher in starch, including some nuts like cashews and vegetables such as corn and potatoes, should be avoided.

You are generally pretty safe starting with meat proteins, fats and most vegetables. Then try adding eggs, almonds, a little low-sugar fruit like berries, grapefruit or apples, then cheese, plain yogurt, and other dairy items one by one and see how it goes.