When was the last time we saw a privateer challenge for wins?

Not sure if your asking about a single event or series but Rick Ryan won the 87 Daytona  SX.

Larry Ward on the Noleen Yamaha comes to mind.  Didn't Ward win a 250 moto on the XXX Yamaha?  Does Roczen on RCH count?  Chad on 22?

If Roczen on RCH and Reed on 22 count, Brayton at Daytona is the more recent private team win.

Before Dowd and Henry were factory riders they won a VERY muddy Hangtown I believe it was?

Who was that privateer that lead Deegan in a Cal. Round ? 

Lance Kobusch

Didn’t Dubach win San Jose SX as a privateer? 

Donny Schmit.  '89

Deegan Senior won a 125 SX on a privateer team. 

Millsaps on the Suzuki in SX. 

Shaun Simpson had been having a hard time in the 2013 GPs on the uncompetitive TM and switched to a private Yamaha team (might even have been the same team as Gifting?) for the last few rounds. At the last round he won the first race and got the overall, as well as standing ovation from the teams in pit row after everything he'd gone through.

Alan King Hangtown national 1984 on a Team Tamm Honda

The most constant over a season has to be Donny in 89 and Dowd in 94

Ty Masterpool with no graphics on his KTM battling at Red Bud. 

Is gifting a privateer? Its not like he shows up in his van with a moto in the back

I would say Lamson in 2000, as he led the points for half the series.

Brown in 2005 was pretty competitive on his Jim's Cycle Sales CRF250.

J-Law could have won Daytona in 2009 (I think that was the year,) if not for Chad Reed. He let him by and settled for 2nd if I remember correctly. Not exactly a full privateer, but it was a satellite team with AM/PM sponsorship. 

Cody cooper getting close to breaking the perfect season at unadilla 08 

What is meant by privateer, a non factory rider, does a b team sponsor  qualify or just a straight up out of own pocket guy. Everyone is sponsored to some degree nowadays and for the last 45 years. Define privateer. There is no such thing as a privateer anymore. Despite his heroics, Marty Moates wasn't even a privateer technically. And Issac Gifting sure as hell isn't either. There is a variety of support available.

If I was any of these new companies, Id sign that guy to ride in the USA.  That Gifting would be a gift.  
prols could could snag him for 400k, living arrangements paid, incentives etc.  1 year contract. 
Practice sx during the winter with no racing but run the outdoors.   Next year, if all is good, let him run sx and outdoors.  
Kid has the heart of a lion. 
Those are the ones you want. 

Masterpool recently. I can’t remember the year. Like last year or before. Wasn’t for wins but he was hounding the podium 3 for a stint there. 

The closest comparison that I think you can make to Gifting's ride would be Jason McCormick's 1999 moto win on a 125 at Washougal.
Like Gifting, McCormick was not on a premiere team and is truly a local (he runs a suspension shop at Washougal MX these days).
 

125cc moto 2 starts at 52:00 in this video...
 

In 2017 in the MX2 class, Julien Lieber held the red plate on a blanco KTM and hardly any sponsors on the shirt.

Kyle Lewis had two wins in 87 125w. 

I always thought that was the last year that he was on the ‘Factory’ team. 

$400K in salary?  Or are you thinking of $400K as the total cost to support him for a year?

was getting ready to make a JL33 post haha dude was so cool. Always liked him.

Larry Ward on the Moto XXX Yamaha is burrrned into my memory. Can remember that Red Bud like it was yesterday.

When he finished ahead of all Suzuki riders after they let him go. 
He used me as a berm in a D23 race at Millville. It was muddy in practice 

It’s my closest brush with fame. 

Kehoe for the dub. '93

Millsaps said his 2013 Rockstar Suzuki bike was not factory at all. Shaun Simpson won Lierop in 2013 for JK Gebben