As far as I know Tesco oats have not been tested for gi.
An average porridge has a gi of about 55 ( but they vary a lot)
Tescos oats have 60.4g carb per 100g .
To find GL
GL = (GI x the amount of carbohydrate) divided by 100
so for 100g of oats ... made into porridge with water
55x60/100 = GL33
do you really mean this amount of oats? it's a big portion, I use a 38g of oats made up to about 250gm with semi-skimmed milk
Here are 2 entries from the GI index database for comparison
Porridge, jumbo oats (Sainsbury's, UK), consumed with 125 mL semi-skimmed milk: GI 40 total weight 250g GL 9
Porridge, small oats (Sainsbury's, UK), consumed with 125 mL semi-skimmed milk GI 61 total weight 250g GL 14