To solve this optimization problem using Gurobi, we need to define the variables, objective function, and constraints according to the given description.

### Variables Definition:
- Let `x0` represent the quantity of milligrams of vitamin E.
- Let `x1` represent the quantity of milligrams of potassium.
- Let `x2` represent the quantity of milligrams of vitamin B9.
- Let `x3` represent the quantity of milligrams of zinc.

### Objective Function:
The objective is to maximize `6.98*x0 + 2.26*x1 + 3.5*x2 + 2.08*x3`.

### Constraints:
Based on the description, we have several constraints:

1. Energy stability index for vitamin E: `6*x0`
2. Muscle growth index for vitamin E: `3*x0`
3. Energy stability index for potassium: `8*x1`
4. Muscle growth index for potassium: `11*x1`
5. Energy stability index for vitamin B9: `5*x2`
6. Muscle growth index for vitamin B9: `2*x2`
7. Energy stability index for zinc: `5*x3`
8. Muscle growth index for zinc: `1*x3`

And the combined constraints:
- `11*x1 + 1*x3 >= 35`
- `2*x2 + 1*x3 >= 50`
- `11*x1 + 2*x2 + 1*x3 >= 42`
- `5*x2 + 5*x3 <= 146`
- `6*x0 + 8*x1 <= 167`
- `6*x0 + 5*x2 <= 148`
- `6*x0 + 8*x1 + 5*x3 <= 121`
- `6*x0 + 8*x1 + 5*x2 + 5*x3 <= 121`
- `3*x0 + 11*x1 <= 276`
- `3*x0 + 1*x3 <= 179`
- `11*x1 + 2*x2 <= 117`
- `2*x2 + 1*x3 <= 152`
- `3*x0 + 11*x1 + 2*x2 + 1*x3 <= 152`

Given that all variables can be fractional, we don't need to specify them as integers.

Here is the Gurobi code for this problem:

```python
from gurobipy import *

# Create a new model
m = Model("Optimization_Model")

# Define variables
x0 = m.addVar(name="Vitamin_E", lb=0)
x1 = m.addVar(name="Potassium", lb=0)
x2 = m.addVar(name="Vitamin_B9", lb=0)
x3 = m.addVar(name="Zinc", lb=0)

# Set the objective function
m.setObjective(6.98*x0 + 2.26*x1 + 3.5*x2 + 2.08*x3, GRB.MAXIMIZE)

# Add constraints
m.addConstr(11*x1 + 1*x3 >= 35)
m.addConstr(2*x2 + 1*x3 >= 50)
m.addConstr(11*x1 + 2*x2 + 1*x3 >= 42)
m.addConstr(5*x2 + 5*x3 <= 146)
m.addConstr(6*x0 + 8*x1 <= 167)
m.addConstr(6*x0 + 5*x2 <= 148)
m.addConstr(6*x0 + 8*x1 + 5*x3 <= 121)
m.addConstr(6*x0 + 8*x1 + 5*x2 + 5*x3 <= 121)
m.addConstr(3*x0 + 11*x1 <= 276)
m.addConstr(3*x0 + 1*x3 <= 179)
m.addConstr(11*x1 + 2*x2 <= 117)
m.addConstr(2*x2 + 1*x3 <= 152)
m.addConstr(3*x0 + 11*x1 + 2*x2 + 1*x3 <= 152)

# Optimize the model
m.optimize()

# Print solution
if m.status == GRB.OPTIMAL:
    print("Optimal solution found:")
    print(f"Vitamin E: {x0.x}")
    print(f"Potassium: {x1.x}")
    print(f"Vitamin B9: {x2.x}")
    print(f"Zinc: {x3.x}")
    print(f"Objective function value: {m.objVal}")
else:
    print("No optimal solution found.")
```