To solve the given optimization problem, we first need to convert the natural language description into a symbolic representation. This involves defining variables and formulating the objective function and constraints using these variables.

Let's denote:
- \(x_1\) as the number of bags of Trail Mix A purchased,
- \(x_2\) as the number of bags of Trail Mix B purchased.

The objective is to minimize the total cost, which can be represented as \(5x_1 + 8x_2\), since each bag of Trail Mix A costs $5 and each bag of Trail Mix B costs $8.

The constraints based on the problem description are:
1. The hiker wants at least 20 units of almonds: Since both mixes contain 2 units of almonds per bag, this constraint can be represented as \(2x_1 + 2x_2 \geq 20\).
2. The hiker wants at least 15 units of chocolate chips: Given that Trail Mix A contains 1 unit and Trail Mix B contains 3 units of chocolate chips per bag, this constraint is \(x_1 + 3x_2 \geq 15\).

Additionally, since the number of bags cannot be negative, we have:
- \(x_1 \geq 0\)
- \(x_2 \geq 0\)

Thus, the symbolic representation of the problem is:

```json
{
    'sym_variables': [('x1', 'number of bags of Trail Mix A'), ('x2', 'number of bags of Trail Mix B')],
    'objective_function': '5*x1 + 8*x2',
    'constraints': ['2*x1 + 2*x2 >= 20', 'x1 + 3*x2 >= 15', 'x1 >= 0', 'x2 >= 0']
}
```

To solve this problem using Gurobi in Python, we can use the following code:

```python
from gurobipy import *

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

# Define variables
x1 = m.addVar(name="Trail_Mix_A", vtype=GRB.CONTINUOUS, lb=0)
x2 = m.addVar(name="Trail_Mix_B", vtype=GRB.CONTINUOUS, lb=0)

# Set the objective function
m.setObjective(5*x1 + 8*x2, GRB.MINIMIZE)

# Add constraints
m.addConstr(2*x1 + 2*x2 >= 20, name="Almonds_Constraint")
m.addConstr(x1 + 3*x2 >= 15, name="Chocolate_Chips_Constraint")

# Optimize the model
m.optimize()

# Print the solution
if m.status == GRB.OPTIMAL:
    print("Optimal solution found:")
    print(f"Number of bags of Trail Mix A: {x1.x}")
    print(f"Number of bags of Trail Mix B: {x2.x}")
    print(f"Total cost: ${5*x1.x + 8*x2.x:.2f}")
else:
    print("No optimal solution found")
```