gegenteilig
opposite
Representing or expressing the exact opposite of something; contrary.
Er vertrat eine gegenteilige Meinung und stimmte gegen den Vorschlag.
He held the opposite opinion and voted against the proposal.

opposite
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opposite
Representing or expressing the exact opposite of something; contrary.
Er vertrat eine gegenteilige Meinung und stimmte gegen den Vorschlag.
He held the opposite opinion and voted against the proposal.

'Gegenteilig' (opposite/contrary) is the adjective derived from the noun 'das Gegenteil' (the opposite). If you know the noun, you can easily work out the meaning of the adjective.
Seine Aussage war das Gegenteil von dem, was er gestern gesagt hat.
His statement was the opposite of what he said yesterday.
This adjective is often combined with abstract nouns such as 'Meinung' (opinion), 'Effekt' (effect), 'Wirkung' (effect/impact), 'Aussage' (statement), or 'Ergebnis' (result).
Die Maßnahme hatte den gegenteiligen Effekt, den wir erwartet hatten.
The measure had the opposite effect from the one we had expected.
'Gegenteilig' (opposite/contrary) can stand before a noun as an adjective or be used as an adverb to describe a verb or the whole sentence. The form of the word does not change.
Sie äußerte eine gegenteilige Ansicht.
She expressed an opposite view.
immediate / same
'gleich' and 'gegenteilig' form a contrast. 'gleich' means that two things match or are identical. 'gegenteilig' means that something goes in exactly the other direction or expresses the opposite. 'gegenteilig' fits especially well with statements, reactions, results, or instructions. If two pieces of information are 'gleich', they confirm each other. If they are 'gegenteilig', they clearly contradict each other.

Zum Schutz der Pflanzen hat der Gärtner eine gegenteilige Methode ausprobiert.
To protect the plants, the gardener tried the opposite method.
Vor dem Wochenende waren die Preise im Laden noch gleich.
Before the weekend, the prices in the store were still the same.