Conclusion First: I don't think Ghost Walk maintains your normal forms movement. But, it is certainly unclear, and 100% a DM's Decision.
Seems clear that there are two different ways to have additional movement. By transformation where your entire mode of movement changes or by being granted something extra that adds to your modes of movement.
Ghost Walk TCE p62 13th-level Phantom feature
You can phase partially into the realm of the dead, becoming like a
ghost. As a bonus action, you assume a spectral form. While in this
form, you have a flying speed of 10 feet, you can hover, and attack
rolls have disadvantage against you. You can also move through
creatures and objects as if they were difficult terrain, but you take
1d10 force damage if you end your turn inside a creature or an object.
Becoming like a Ghost, and Assuming a Spectral form, both imply that your form changes as you step into the realm of the dead.
As others have said, Ghosts and Specters have 0 movement and a fly speed.
Ghost MM p147 0 ft., fly 40 ft. (hover)
Specter MM p279 0 ft., fly 50 ft. (hover)
Fly tells us that we Gain a Flying speed
Fly PHB p243 The target gains a flying speed of 60
Gaseous Form that we are transformed, makes sure to tell us that we ONLY have the fly speed of 10
Gaseous Form PHB p244 You transform a willing creature you touch,
along with everything it's wearing and carrying, into a misty cloud
for the duration. While in this form, the target's only method of
movement is a flying speed of 10 feet.
Perhaps this particular makes things most unclear, as this is the first time the movement is a hindrance, being that it is less than what we assume a creatures normal mode of movement would be.
Wind Walk PHB p288 You and up to ten willing creatures you can see
within range assume a gaseous form for the duration, appearing as
wisps of cloud. While in this cloud form, a creature has a flying
speed of 300 feet
Wind Walk once again changes our form into something 'other' and while it doesn't tell us only, it is assumed you would only fly with it being so high speed.
Spider Climb applies the GAIN clearly not changing our mode, only the kinds of walking
Spider Climb PHB p277 Until the spell ends, one willing creature you
touch gains the ability to move up, down, and across vertical surfaces
and upside down along ceilings, while leaving its hands free. The
target also gains a climbing speed equal to its walking speed.
This gain doesn't effect our normal walking speed, it is in fact equal to it.
At 20th level, you gain a feature granted to you by your Sacred Oath.
Avenging Angel PHB p87 At 20th level, you can assume the form of an
angelic avenger. Using your action, you undergo a transformation. For
1 hour, you gain the following benefits:
Wings sprout from your back and grant you a flying speed of 60 feet.
Avenging Angel transforms us, but the transformation is Wings SPROUTING and that we are GRANTED or gain a flying speed. Once again implying addition.
I think this is vague. Most of the transformation language is the same as the Ghost Walk language... Ghost Walk speaks as we HAVE a flying speed, not that we GAIN it. Ghosts and Specters ONLY have Fly Speeds.
BUT... Gaseous form is unique in the transformations that say we ONLY HAVE... the HAVE language is important, but this is a unique time that they use the ONLY. I don't think any other transformation uses it primarily because it was recognized as superfluous