günstig
cheap
Having a low price; inexpensive, not expensive.
Auf dem Markt habe ich sehr günstiges Gemüse gekauft.
I bought very cheap vegetables at the market.

cheap / favorable
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cheap
Having a low price; inexpensive, not expensive.
Auf dem Markt habe ich sehr günstiges Gemüse gekauft.
I bought very cheap vegetables at the market.

The adjective 'günstig' (cheap/favorable) comes from the noun 'die Gunst' (favor, goodwill). A 'günstiger Preis' (cheap price) is therefore, figuratively, a 'benevolent' price, and a 'günstiger Moment' (favorable moment) is a moment 'favored' by fate.
Er versuchte, die Gunst seines Chefs zu gewinnen.
He tried to win his boss's favor.
When 'günstig' is used in connection with buying, prices, or offers, it always means 'inexpensive' or 'not expensive'.
Ich habe ein sehr günstiges Flugticket nach Spanien gefunden.
I found a very cheap plane ticket to Spain.
In connection with abstract nouns like 'Moment' (moment), 'Gelegenheit' (opportunity), 'Wetter' (weather), or 'Bedingungen' (conditions), 'günstig' always means 'favorable' or 'suitable'.
Die Wettervorhersage für unsere Wanderung ist günstig.
The weather forecast for our hike is favorable.
When referring to places, apartments, or transport connections, 'günstig' means 'well located' or 'easy to reach'. It often appears together with verbs like 'liegen' (to be located) or 'wohnen' (to live).
Das Hotel liegt verkehrstechnisch sehr günstig in der Nähe des Hauptbahnhofs.
The hotel is very conveniently located in terms of transport, near the main train station.
happy / lucky
'glücklich' and 'günstig' can both express that something turns out well for a situation. Here, 'günstig' means favorable, suitable, or helpful for a plan or purpose. 'glücklich' emphasizes more that a good outcome also has to do with luck or a fortunate coincidence. Learners may confuse the words because both describe something positive. For conditions, times, or circumstances, 'günstig' is usually more precise; for a good outcome due to luck, 'glücklich' fits better.

Am Montag war der Zeitpunkt für das Gespräch günstig, weil alle im Büro waren.
Monday was a favorable time for the conversation because everyone was in the office.
Er hat den Unfall glücklich überstanden und ist nur leicht verletzt.
He survived the accident luckily and was only slightly injured.