It's a free and open-source app. No ads, nothing shady or obscure. If you have any doubt, feel free to ask or browse the source code.
Donations are the only source of income of ScreenToGif. If you would like to support this project then please consider donating.
There's 4 ways to donate, via Patreon, Paypal, Coinbase or Steam (it's possible to donate games or credits).