sich etwas gönnen
to treat oneself
To do something nice for oneself or allow oneself a special pleasure, a break, or a luxury.
Nach der anstrengenden Woche gönne ich mir ein langes Bad.
After the exhausting week, I'm treating myself to a long bath.

to treat oneself / to not begrudge
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to treat oneself
To do something nice for oneself or allow oneself a special pleasure, a break, or a luxury.
Nach der anstrengenden Woche gönne ich mir ein langes Bad.
After the exhausting week, I'm treating myself to a long bath.

'Gönnen' is a verb that is almost always used with a person in the dative and a thing in the accusative. The structure is: someone (subject) grants/treats someone (dative) to something (accusative).
Ich gönne meiner Kollegin (Dativ) die Beförderung (Akkusativ).
I don't begrudge my colleague (dative) the promotion (accusative).
When you do something nice for yourself, 'gönnen' is used reflexively. The person then appears as a reflexive pronoun in the dative: 'sich (dative) etwas (accusative) gönnen' (to treat oneself to something).
Heute Abend gönne ich mir ein gutes Buch und eine Tasse Tee.
Tonight I'm treating myself to a good book and a cup of tea.
When you treat yourself to something, it is often a small moment of luxury or enjoyment in everyday life. Typical examples are: 'sich eine Pause gönnen' (to treat oneself to a break), 'sich ein Stück Schokolade gönnen' (to treat oneself to a piece of chocolate), or 'sich einen freien Tag gönnen' (to treat oneself to a day off).
Nach dem Sport gönne ich mir immer eine lange, heiße Dusche.
After exercising, I always treat myself to a long, hot shower.
'Jemandem etwas gönnen' (to not begrudge someone something) is the positive opposite of envy. It expresses that you are sincerely happy about another person's success or happiness and grant it to them wholeheartedly.
Er hat so viel gelernt, ich gönne ihm die gute Note von Herzen.
He studied so much; I truly don't begrudge him the good grade.
to wish
'wünschen' and 'gönnen' both have to do with something good that someone should have or experience. 'wünschen' means that you would like to have something or that you hope for something good for another person. 'gönnen' is stronger: you think it is right and good that someone gets something nice, and you are not envious. With 'sich etwas gönnen', you mean that you deliberately allow yourself something pleasant. The words can be confused because both sound positive. But 'Ich wünsche dir Urlaub' sounds like a good wish; 'Ich gönne dir den Urlaub' emphasizes that you deserve the vacation.

Nach der langen Woche gönnt sich Paul einen ruhigen Abend.
After the long week, Paul treats himself to a quiet evening.
Ich wünsche meiner Freundin viel Erfolg für die Prüfung.
I wish my friend lots of success on the exam.