Let's get to work on some basics, then. Stings: do we want to add some piercing damage (the standard doesn't have any)? Maybe 1d4 or 1d6 per sting? Upsizing the acid from 1 at Diminutive by the usual table is 3d6, but that feels a bit big, especially if we don't want to make these too strong compared to gammaroids. Maybe 1d8 acid?
I'm leaning towards just having it do damage via its secretions (e.g. acid plus poison). If we give it a natural weapon damage it'll get a Strength bonus of +11 on it, which doesn't feel right - it'll likely mean it does more damage from impact than its corrosive enzymes do.
Unless we give it some special rule where it doesn't add its Strength bonus to its sting damage I suppose.
Simply upsizing the Standard Gossamer's 1 point acid doesn't work very well because they've got a single attack, while the Noble Gossamer has twenty.
Let's try comparing some numbers.
The original Noble Gossamer has a 20d6 damage potential (average 70), while the original Gammaroid had 20d6+60d4 (average 220), which is about a third as much.
Our Gammaroid conversion averages 154, so it's one of the rare cases where our 3E version does less damage than the original! Of course, that's ignoring Power Attack - it could easily use 50 points of Power Attack and increase its average damage to 300-odd without reducing its likelihood of hitting most targets.
If we use that 300-point average Gammaroid damage as a baseline, giving the Noble Gossamer a 1d8 acid sting would fit, but it just feels a little feeble. Especially when you consider any creature with acid resistance 10 either naturally or from a spell would be effectively immune to its attacks.
Maybe we should rekindle our love of the dodecahedron and make it 1d12?
Alternatively (or additionally) we might also give it Improved Grapple and the ability to combine its acid damage with constrict attacks (e.g. one sting does 1d8, two stings do 2d8, et cetera) rather than having them roll damage separately.