The employer could gross up your bonus if they wanted to, but that is up to them.
After tax reform, relocation expenses are not tax deductible to the employer or employee. It is treated as if you received a cash bonus and is taxable as such, no matter what you used it for.
If your expenses were, for example $5000, the employer could pay you $8000 to cover the taxes, but that would be up to them. What did you agree to in your contract?
