gnadenlos
merciless
Without mercy, pity, or compassion; very harsh and unyielding.
Der Diktator verfolgte seine Gegner gnadenlos.
The dictator mercilessly persecuted his opponents.

merciless / extremely
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merciless
Without mercy, pity, or compassion; very harsh and unyielding.
Der Diktator verfolgte seine Gegner gnadenlos.
The dictator mercilessly persecuted his opponents.

The word is made up of 'Gnade' (mercy) and the ending '-los' (-less), which means 'without'. So 'gnadenlos' (merciless) literally means 'without mercy'.
Die Mannschaft war ihrem Gegner gnadenlos überlegen.
The team was mercilessly superior to its opponent.
In colloquial language, 'gnadenlos' (extremely) is often used as an intensifying word for adjectives, similar to 'extrem' (extremely) or 'total' (totally). It usually has a negative or ironic tone.
Die Preise in diesem Restaurant sind gnadenlos hoch.
The prices in this restaurant are extremely high.
In its main meaning, 'gnadenlos' (merciless) is often used as an adverb to describe an unyielding action. Typical verbs are 'ausnutzen' (to exploit), 'zuschlagen' (to strike), or 'durchsetzen' (to enforce).
Er hat die Situation gnadenlos ausgenutzt.
He mercilessly exploited the situation.
completely
'völlig' and 'gnadenlos' can both make a statement stronger. 'völlig' is neutral and simply means 'entirely' or 'completely'. 'gnadenlos' is also used colloquially as an intensifier, but it sounds harsher and often negative. It fits well when something seems extreme, unpleasant, or ruthless. You can say 'völlig müde' (completely tired) in any normal situation. 'Gnadenlos müde' sounds stronger and more casual, almost like an exaggeration.

Wegen der Hitze war der Weg zum Bahnhof gnadenlos anstrengend.
Because of the heat, the walk to the station was extremely exhausting.
Am Bahnhof war sie völlig verwirrt, weil der Zug nicht angezeigt wurde.
At the station, she was completely confused because the train was not displayed.