Sorry you feel so rubbish about the gain... It's not a fun thing to find when you get on the scales.
Like the others have said, keep free treats around the house. Another tip would be to buy things that are synned, but write the syns on them before you put them in the cupboard/fridge. I know plenty of people who have a 'syn box' and they keep things like Freddos, Kit Kats, the mini packets of Haribo, or some biscuits, and write the syns for each thing on the packet (5 syns for a Freddo, 5.5 for a 2 finger Kit Kat, 1 for each Rich Tea Finger, 4.5 syns for 25g of Haribo, etc.) and that way they know how many syns they're eating. Check the Syns Online section on the website before you do your shop to find some low syn treats (or buy the thin purple low syn snack directory from group) That way you don't end up buying something that's really high in syns. There are usually free alternatives that you can make, but for me personally, I prefer to syn the real thing.
One of my favourite treats are the mini Twister ice lollies... 2.5 syns per lolly, so if I've had very few syns, and really crave something sweet (or cold on a hot day!) I can have one or two of those.
Keep a notepad close to you so you can note down any syns through out the day, and then you now what you can have in the evenings.
Keeping a food diary is key for me. Once I stop writing everything down, I lose track completely and end up with a gain.
Good luck for the rest of the week hun, don't let that gain get to you and send you off track completely. What's done is done, there's no way to change it now. All you can do is to start fresh, and try to write everything down