Getting a gold medal in an experiment not only requires you complete the "bronze medal objective," but also obtain a certain score as well. The best advice is to try and obtain the largest multiplier possible and pick up corn! I like to complete the bronze objective early in the level, then focus on getting my score up by collecting corn. You must gold all levels of all laboratories in the game, except obviously for the PDLC ones.
One technique I found that made this game easy was to try and collect magnets. When you use a magnet, all the corn you pass is collected automatically. While the magnet is active, focus on avoiding obstacles, moving fast, and most importantly racking up power-downs to increase your multiplier. If you do not have any magnet, I recommend going slow and try to find a magnet. This is basically what I did for all levels (that actually have magnets, some do not) and had little trouble getting gold on all levels.
A tip for the Aquarium experiments: The developers seems to have made these new levels noticeably harder than the rest of the game after they patched it. Many of the levels I found impossible to complete unless you realize the importance of one key collectible: the pearl! Many levels you just need really to ignore the corn and look for power-downs and try to pick up a pearl when you have a good multiplier. The base value of a pearl is 1000, so you can see how they really help you rack up score fast, compared to trying to collect corn. Once I realized this, these levels became easy, but involved some luck to get enough pearls to spawn in the level.