This seems to depend on what kind of corporation you have set up.
If you're set up as a sole proprietor, then the Solo 401k contributions, whether employee or employer, will be deducted from your gross income. Thus they don't reduce it.
If you're set up as an S-Corp, then the employer contributions, similar to large employer contributions, will be deducted from wages, and won't show up in Box 1 on your W-2, so they would reduce your gross income. (Note, employee contributions also would go away from Box 1, but would still be in Box 3 and 5 for FICA/payroll tax purposes).
This is nicely discussed in detail here.
The IRS page that discusses this in more (harder to understand) detail is here.
Separately, I think a discussion of "Gross Income" is merited, as it has a special definition for sole proprietorships.
The IRS defines it in publication 501 as:
Gross income. Gross income is all income you receive in the form of money, goods, property, and services that is not exempt from tax. If you are married and live with your spouse in a community property state, half of any income defined by state law as community income may be considered yours. For a list of community property states, see Community property states under Married Filing Separately, later.
Self-employed persons. If you are self-employed in a business that provides services (where products are not a factor), your gross income from that business is the gross receipts. If you are self-employed in a business involving manufacturing, merchandising, or mining, your gross income from that business is the total sales minus the cost of goods sold. In either case, you must add any income from investments and from incidental or outside operations or sources.
So I think that regardless of 401(k) contributions, your gross income is your gross receipts (if you're a contractor, it's probably the total listed on your 1099(s)).