Do you want to check the results of assembly before transformation? Or before sequencing? If it’s before the transformation, it would be difficult to get the desired results. If you want to check before sequencing, then you can. As transformation followed by growth in bacteria colonies will amplify the copies of the plasmid, which will help in getting the bands on gel electrophoresis.

Get the non-assembled plasmid (better the uncut/circular plasmid) and extract the assembled plasmid after transformation. Run both the plasmids in side-by-side wells with your ladder. You could differentiate the bands of size 3.8 kb and 5 kb easily if the assembled plasmid is successful.

Well, after Gibson assembly, there should be 3 products in the mixture: insert, linearized vector and construct (vector + insert). But if only a small amount of insert ligate into the linearized vector, you may not see much on the gel. Maybe you can try to do PCR with both primers that anneal on the vector and can amplify the insert producing fragments as below:

partial vector - insert - partial vector

If amplification shows the correct size, then can proceed with transformation, if amplification shows no band or smaller fragment means failed ligation. I've never try this before, but theoretically it should be feasible. It will require about the same amount of time as when you do transformation but its up to you to decide between time vs cost.

Gibson assemblies can be directly run on a gel for diagnostic analysis. Take half of your 20 uL reaction and run it on a gel. As a control use your DNA input with water as opposed to enzyme mix. A successful assembly should deplete the smaller bands of your individual components, and result in 1 or multiple bands running at larger sizes.