Halloween Gambit is absolutely sound.
What well known gambits are unsound?

@ Pacifique: The Q was not for me, but I will try answering it.
The critical line seems to be 7.0-0 Ngxe5 8.Nxe5 Nxe5 9.Na3! Be7 10.Nb5 0-0 11.Nxd4 Nxc4 12.b3 Bf6 (12...Nd6 is playable, but somewhat passive) 13.e3! (13.bc4 Bxd4?! 14.Rb1, as played in a recent blitz game between Chucky and Moro, should be better for white a bit, but 13...Qxd4 is an equalizer) 13..c5 14.bc4 cd4 15.ed4 Bxd4 (15...Qxd4 16.Qxd4 Bxd4 17.Rb1 is comfortably better for white) 16.Rb1 Be6! when white is certainly more active, but the material is quite reduced, and I doubt if white's advantage can spell something more than the better part of a draw.
7.0-0 Ngxe5 8.Nxe5 Nxe5 9.Na3! seems to be the most testing indeed. What about 10.Nb5 0-0 11.Nxd4 Nxc4 12.b3 Ne5 (instead of 12...Bf6) 13.Bb2 Bf6 ?

Pacifique if you can be at all objective about the Ponziani position and the lines which follow you would have to say Black is defendings all the way--am I correct?
There are ways for White to improve even earlier which are noted in my book but in general this position with a White spatial advantage is no fun for Black at all.
Even if white has some small advantage the draw is the most plausible result between more or less equal players.

every gambit is sound, as long as you have good results with it vs. players on your level or better player...

Sorry, why would white castle on the eighth move? The idea is that white is trying to win, not lose. I think my king stayed in the centre for quite a few more moves. There are plenty of things white would do rather than castle: one might be a3, which is probably in character for me.

7. Bg5 looks interesting too. Maybe that could be played and then white would have a pleasant choice after Be7.

Oh yes, e3 then, same difference. I've had a long week.
And what kind of problems Black should have after 6.e3 Bg4 ?

In the above position [if it can be reached after 7. ...Bf5] does not White threaten Rd1 putting a lot of pressure on the black position?
[I don't know the answer--just curious]

7. Bg5 looks interesting too. Maybe that could be played and then white would have a pleasant choice after Be7.
7.Bg5 Qd7!

Pacificque I see you cannot be objective about the Ponziani space advantage position where I say it is White who has the chances and fun!

7.0-0 Ngxe5 8.Nxe5 Nxe5 9.Na3! seems to be the most testing indeed. What about 10.Nb5 0-0 11.Nxd4 Nxc4 12.b3 Ne5 (instead of 12...Bf6) 13.Bb2 Bf6 ?
This is OK of course, but still white should have a regular small edge following 14.Rc1 or 14.Qc2. To be honest, I never bothered to find the actual magnitude of the problem, since I'm perfectly happy with 5.a3! which I have analysed quite thoroughly (despite the fact that i usually play 2.Nf3 first, when... no Albin or regular Chigorin (which annoys me much more than the Albin).
14.Qc2 seems to be more challenging due to 14...c6 15.Rfd1 when black should choose where to place queen (probably b6). But even then I`m not sure how white can exploit this small edge.
On 5.a3 Nge7 are you gonna play 6.b4 or 6.Nbd2 ? The last one seems to be more challenging.
Pacifique if you can be at all objective about the Ponziani position and the lines which follow you would have to say Black is defendings all the way--am I correct?
There are ways for White to improve even earlier which are noted in my book but in general this position with a White spatial advantage is no fun for Black at all.