The weight is fine.
I'd probably go up to 2x normal = 116 HD for advancement.
I'm going to guess around 28 or 29 for CR. Or maybe 27. The gibbering orb also has a lot of not-too-potent attacks; the eye rays probably bump it up a bit, but the noble gossamer has a lot more hp plus a miss chance (though worse AC).
A Gibbering Orb reads a lot nastier to me.
The can use all 36 of its attacks as a standard action, and its eye rays include
disintegrate,
energy drain,
feeblemind,
harm,
horrid wilting and
implosion - all horrible effects which it can use five of against a single target, while our Noble Gossamer can only hit a single Medium opponent with two of its tentacle sting attacks. If could target an opponent with five energy drain eye beams and sap them of 10d4 energy levels - that's something to strike the fear of Appolyon in your PCs!
The Orb also has a fine Spell Resistance of 37 and immunity to critical hits and flanking. It does have much worse saves than our Gossamer, though its superior AC partly balances that out.
It may have better saves, but its low Dex and poor mental stats make the Gossamer very vulnerable to ability damage in comparison to the Orb, who's lowest stat is its 22 in Charisma.
Also, to render a Noble Gossamer's special attacks useless is comparatively easy, requiring a combination of
free action (for the grappling), either
neutralize poison [or
hero's feast] (for the venom) and
resist energy (acid) (for the digestive enzymes). That's two 4th-level spells and a 2nd-level spell, a fairly negligible expenditure for a high-level party.
To safeguard against a Gibbering Orb's eye rays you'd need protection from ALL its eye ray effects, of which it has twenty four to choose from, and even then it can steal the spells and SLAs of anything it has eaten in the last day and use them as its own SLAs, casting two of them
every round.
I'd say the Gossamer is significantly weaker. Maybe somewhere around CR 20-25 if I'm feeling generous?
Put it this way, if it was fighting our CR 25 Hawk Lord conversion in a straight one-on-one melee fight, how would it fare?
The Hawk Lord could use 7 points of Superior Expertise to raise her AC to 66, meaning the Gossamer would have to roll a 20 to hit her, while the Expertise penalty to attack leaves Madame Raptor with melee attacks of +45 (single rapier) and +41 (paired rapiers, secondary claw/claw/bite). That means she still has enough attack bonus to add a 10-point Power Attack and
still be almost guaranteed of hitting the Gossamer with her initial iterative attacks. A back-of-the-envelope calculation indicates she'd do about 170 damage per round to the Gossamer, killing it in 5 rounds or so.
During that time the Noble Gossamer could attack her ten times, needing a 20 to hit, so would have a roughly 40% chance of hitting her at least once. The Noble Gossamer's grapple bonus of +70 is so much higher than the Hawk Lord's +42 it's effectively an auto-grapple with Improved Grab, but the Hawk Lady can simply
greater teleport out of its grip. She'll almost certainly fail her Fort Save and take some Dex damage, but not enough to paralyze her. She'd only lose if the Gossamer got lucky and hit her with multiple stings in the five rounds it has to live.
Or, for a comparison with an SRD monster, how would the Noble Gossamer fare against the CR 20 Tarrasque? (Obviously it'd have to be the previously unknown
Space Tarrasque to actually encounter a Noble Gossamer...

)
Things are better for Gossie. The Gossamer can't paralyze the Tarrasque, because it's immune to ability damage, but it will almost certainly hit (+46 attack vs AC 35) and the Tarrasque's so big it can be attacked by a dozen stings at once (2 stings per 5 ft. versus a 30 ft. Space). So it's doing roughly 12d8 acid damage per round, enough to get through the Tarrasque's 858 hit points (which I will point out is
more than the Gossamer's 725 hp) in sixteen rounds or so. That's forgetting about the Tarrasque's regeneration of 40, which would drop the damage-per-round to 14 and increase the "time to kill" to over 60 rounds - let's assume our Space Tarrasque takes normal damage from Gossamer acid.
Unfortunately, the Tarrasque can tear a Noble Gossamer to pieces in four or five rounds by charging and using full attacks with a 20-point Power Attack (which figures out to at roughly 220 damage-per-round).
Two Noble Gossamers would take twice as long for the Tarrasque to kill and inflict three times as much damage, so are an almost-equal match.
Three Gossamers would almost certainly win the fight at the cost of one dead Noble Gossamer even if the Tarrasque could use its regeneration 40.
The above comparisons are a little unfair, because they're one-on-one, and Noble Gossamers will be more effective when fighting multiple opponents, since it'll allow them to bring into play most or all of their sting attacks.
However, against a typical party of adventurers I'd be reluctant to give it a CR much above 20, maybe 22-23 tops. I'd even consider CR 19, since it's weaker in a mano-a-mano fight against a Tarrasque. (Which'd also be its Hit Dice divided by 3, for what it's worth).