What the heck?

As Unholy, (IL 399) I can hardly do 40k single target in raids, and on the dummy maybe 25k.

I am specced right, gemmed enchanted, etc.
Gnome DKs look pretty cool in the ds teir!
unless you changed something, it looks like you have way too much exp and hit

go check out wowreforge or somethign and see what it says
On my screen you appear to be this mishmash of master frost. If you are wanting unholy you'd definitely have to respec and reforge and so on. Hit has to be a lot higher and your Expertise drops to the bottom. But if you were trying to be unholy with your current stats? YOu'd have a problem and if you're only doing 25k? Prolly not managing your CD's and timmmy enough. You should see more single target as Unholy over frost or at least higher peaks. Frost can pull ahead while Unholy is waiting for CD's to cycle but once those Cooldowns are back like Frenzy and the Gargoyle? They'll be back up or push ahead. Apples to oranges. No matter what though can't say what you're doing wrong while your armory is still frost using 2 morchoks in mish mash of master frost.
Yeah, neither of your specs are Unholy, thus I have to assume that none of your gear is reforged/enchanted/gemmed for Unholy, so it's kinda hard to tell you what you might be doing wrong.

But you have 8.72% Hit, and you only want 8%. You have 27 Expertise, and you only want 26 as Frost. For Unholy, you want to reforge out of Expertise as much as possible, because Expertise has less value to UH than Mastery and Crit. So yeah, with the way you have your gear reforged/enchanted/gemmed right now, I wouldn't expect you to be doing particularly great DPS as Unholy. You want to get to 8% Hit, as close to 0 Exp as possible, and stack Haste. If you have the Gurthalak sword and the 4pc set bonus, you want to stack Mastery after Haste. If you don't have Gurthalak, you want to stack Crit after Haste. I'll try to log out in my PvE gear next time so you can take a look at my reforging.

As for single-target priorities: you want to be cycling your runes via Festering Strike to go back and forth from BBFFUU to DDDDUU, using Outbreak to apply diseases. For the first half of the cycle, you use Death & Decay and Scourge Strike to use up your UH runes, then use 2x Festering Strike to proc Reaping and to add 12 sec to your diseases. For the second half of the rune cycle, you want to use all 4 Death runes and 2 UH runes on Scourge Strike (or DT if it pops). You want to keep Runic Corruption and Dark Transformation up as much as possible via Death Coils. Make sure that your ghoul is always using his Claw ability to maximize DPS. Use AMS when taking magic damage to RP-soak for Summon Gargoyle or for Death Coils for DT. If you have more in-depth questions, feel free to ask.
Yes people, I changed specs since then.