The tank itself i am recommending 10% of the clorox. Let it sit minimum for 20 min. I know it sound scary but it works unless you have allergy to clorox. A lot of people on the forum recommending it. But it would be only for the tank. Wash it thoroughly like million times with hot warm cold water , wipe it , wash it, let it sit in the sun for a few days to neutralize the clorox.
If you don't want to use the clorox you can use 50% vinegar and you really can wash with hand antibacterial soap(foam one) . Just wash it very well,wipe,wash and let it air dry for a few days.
Gravel can you buy new? I would not disinfect the gravel.
Silk and plastic plants soak it in boiling water and 50% vinegar , let it cool down then wash it with hot,worm,cold water really well let it dry out for a few days.
Really not sure how to disinfect the filter. I would afraid to use the same filter again. But actually for 2-2.5 i would not even recommend to have a filter. I don't have filters for any of my tank(2.5; 5 gall tanks). It better to do 1-50 and 1-100% water changes a week for that size of the tank. Or just 100% every 5 days.
Sorry a few more tips
Next time when you buy fish always quarantine it for 2 weeks before you put it in the tank. A lot of times you can bring sick betta from the store. The same with live plant if you ever decide to have them. Plants also can carry diseases. Just put it in the water NO water conditioner for a few weeks.
Be careful when you buy fish at the store. If people don't have a lot of experience i always recommend to wait until new shipment arrive or get it from the breeder. Let me know if you ever want to get it from the breeder . I know a few people on this forum i can recommend to you. A lot of time when you look at the fish in the store you cant say that it sick but still i want to give a few tips that is very easy to recognize. A few symptoms: fish that lay at the bottom, bloated, has white cottony patches on the body, hanging in one spot, one or both gills do not close all the way and stay open, red sores, cloudy eyes, darting, white spots all over the body, ragged , split tail.
Good luck. Let me know if you need any help . I am glad you considerate to rescue another cute betta :)