Put a grill pan on the stove over high heat. Grease with butter when hot then put the 4 slices of bread (maybe do 2 slices first if grill pan is not big enough).
When bread slices are browned, take it out of the pan, set aside on a plate. Apply more butter on the grill pan and put the other 4 slices of bread.
Meanwhile, spread about a tablespoon of simple pesto on the toasted side of each bread. Put a slice each of gouda and cheddar cheese on top of each slice.
Take out the slices of bread that's on the grill, set aside. Butter the pan again then put the slices of bread with the pesto and cheese back on the pan and cover each bread with the toasted side (hot part) of the remaining bread slices to make a sandwich.
Using a flat angled metal spatula, press on each sandwich to brown the bread and melt the cheese inside. Take bread out of the pan, apply more butter to the pan and put the sandwiches back to grill the other side. Press down with an angled spatula.
if you have a grilled cheese sandwich maker, it would be easy-peasy. I don't so I do it this way. :D