Gas match field will let you create a new field and gas analysis will do a specific calculation, such as find the gradient or curvature. It's similar to the vdb analysis. Then gas calculate lets you do math operations, such as combining fields by adding or multiplying, etc. So if you had a FLIP sim, you could use a gas match field to create a new field, using the flip surface field as reference. Then you could calculate the gradient of the surface, and store it in that newly created field. Make sure to pick the right class of field for the new field, based on the operation you'll do with the gas analysis. Ie: to store a gradient calculation, it needs to be vector. Now you have a vector field, based on the gradient of the flip surface. You can use a gas calculate or gas linear combination (preferable for simple operations) to add that field to vel.
Also, there's a masterclass called 'building fluid solvers" or something like that. definitely worth checking out.