Guppies as a species also unfortunately often come with "baggage" like parasites etc, so hoping this is not the case but if they don't perk up you may need to look into their health as well.
Before you go out and get more guppies, from what I understand gouramis are best suited to relatively soft water, while guppies are suited to hard or very hard water, so you may have a conflict there. As a beginning aquarist, it is always easiest to try and match your fish to the hardness of your water, instead of taking on the stress of trying to modify your water to suit your fish. If you tested your gh and kh, go ahead and post them here and you will get a lot of advice on what fish are best suited for your water parameters.
Also, I recommend that you keep a very close eye on your ammonia and nitrites for the next few weeks (testing as often as twice a day). If they begin to rise, you will immediately need to do a 50% water change to alleviate chemical stress in your fish. After the levels are no longer spiking you will have an "established" tank, and you will not have to be constantly measuring the values.
By any chance did you use old filter media that had been running recently when you set up the filter? Or did you use all new gravel and media and decorations? If the latter, you are likely going to have some form of nitrogen cycle occur now that you have added fish, since even though the test values read zero to start with, the tank may not have experienced as much ammonia as what the fish are now producing and the bacteria may need a while to catch up.
However, fear not! There are so many wonderful knowledgeable people on these forums willing to help! Keep us posted on how everything goes and let us know what your gh and kh values are and we can help with what fish may be best to help those timid guppies out!
You need to test the water every day, a test kit of your own is a lot better than going to the shop every day. Those with liquid reagents and test tubes are better than strips - and they include the vital ammonia test which isn't on 5 or 6 in one strips.
When you washed the old gravel you killed a lot of the bacteria in there, I'm afraid.
Just in case the shop didn't really understand what a fish-in cycling involves:
Fish excrete ammonia, it's their version of urine. Ammonia is toxic to fish. In a mature tank there are bacteria which eat ammonia and they turn it into nitrite, which is also toxic to fish. In a mature tank, another species of bacteria eat this nitrite and they turn it into nitrate. Nitrate isn't as toxic as the other two and we remove it by doing water changes.
In a new tank, there are only a tiny amount of these bacteria from the tap water but not nearly enough. You have to grow a lot more of them. The ammonia eaters grow first as the fish continually excrete it. it is your job to do a water change every time there is a reading above zero or the fish will suffer (many shops tell you to let the ammonia level get quite high but this will burn the fish's skin and gills making it harder for then to get oxygen through their gills). After a week or two enough ammonia eaters will have grown to make enough nitrite for that to show in the test. Now the nitrite eaters can start to grow. But tey always lag behind the ammonia eaters. Water changes must be done every time there is nitrite above zero. Nitrite binds to the fish's red blood cells preventing oxygen from binding. It basically does the same to fish as carbon monoxide does to us. Again the shop may have told you to let nitrite get high but this will damage the fish - it's like us sitting in a room with a faulty gas appliance.
This is why you need to test every day and do a water change if either/or ammonia and nitrite are higher than zero. The amount of water changed needs to be enough to get the level back down to zero.
There are things you can do to help.
As Colin had already mentioned, feed only 2 or 3 times a week. Less food = less ammonia.
Get a lot of live plants, especially floating plants. These take up ammonia as fertiliser and they don't turn it into nitrite or nitrate.
Add Tetra Safe Start. This is one of the two most highly recommended bottled bacteria products (the other one isn't easy to get in the UK)
Use Seachem Prime water conditioner. This detoxifies ammonia and nitrite for 24 hours and will protect the fish between daily water changes.
Very nice presentation of ideas. I love this Forum!
Definitely get your own test kits, it is an investment that you will not regret. Understanding the underwater world that your fishes are living in is the key to your success in providing a safe and healthy aquarium.