It's true that Democrats have done very well in legislative and congressional special elections this year and that should be encouraging for them in 2018. A Republican being in the White House tells us all we need to know that this election should be a Democratic wave. You can stop reading here if that's all you need to know.
If not, here are some caveats. Don't compare the special election results to Trump-Clinton numbers. The biggest reason is that's a Presidential race between two candidates that don't live in the district, or in this case state, and this is a local race. In November Republicans rarely saw big gains in districts Trump improved over Romney and Democrats didn't see big gains in most district that Clinton improved over Barack Obama. Congressional numbers are a better barometer.
Ryan Zinke won the district by 15.6% in 2016. So a 7 point win for Gianforte shows a Democratic gain. One problem with that is that Zinke was an incumbent. This, like all special elections, had no incumbent. A party will almost always do better in an open seat race than when going up against an incumbent.
One thing that may be encouraging for Republicans is that the turn out in the special election was higher than the 2014 mid-term. Special elections are usually lower enthusiasm lower vote total affairs. Democrats are hoping for low Republican turn out but that doesn't appear to be the case in this election. If Democrats can't win districts with mid-term turnouts that could bode ill for them in 2018.
Trump won this state by a large margin and that's caused some Democrats to dismiss the district as one they won't have to win in 2018. When looking at congressional results, it is. Republicans won 217 seats by 13% or more. If Democrats were to win all of those seats they'd have the 24 they'd need to win back the House. Of course, planning on winning just those exact seats. They'll need to target a much broader group.
There are another 17 seats Republicans won by 13.4% - 15.6%. This is a good group to look at but they also look at any seat the GOP won by 20% or less that's open. A Republican incumbent who won by 12% may be harder to beat than winning an open seat that the GOP won by 20%. Zinke won MT-AL by 15.6%. That puts this seat at the upper limit Democrats should look at but it was also an open seat and that makes it far more likely to flip. Democrats can't dismiss any open seats in 2018.