Let's calculate the cost price and selling price step by step and then determine the percentage gain or loss.

1. Cost Price (CP) of 100 oranges at 5 for N1.20:

CP = Number of oranges * Cost per orange

CP = 100 * (N1.20 / 5)

CP = N24

2. Number of oranges sold after 20 got spoilt:

Remaining oranges = Total oranges - Spoilt oranges

Remaining oranges = 100 - 20

Remaining oranges = 80

3. Selling Price (SP) of the remaining oranges at 4 for N1.50:

SP = Number of oranges * Selling price per orange

SP = 80 * (N1.50 / 4)

SP = N30

Now, let's calculate the percentage gain or loss:

Gain or Loss = Selling Price - Cost Price

Gain or Loss = N30 - N24

Gain or Loss = N6

Percentage Gain or Loss = (Gain or Loss / Cost Price) * 100

Percentage Gain or Loss = (N6 / N24) * 100

Percentage Gain or Loss = 25%

So, the trader incurred a 25% gain.