Die Preisgarantie gilt bis Dezember.

Because Preisgarantie is a feminine noun.

A very useful rule in German is:

• the last part of a compound noun decides its gender

Here, the compound is:

• Preis = price
• Garantie = guarantee

Since Garantie is feminine, the whole compound is feminine:

• die Garantie
• die Preisgarantie

So the sentence starts with Die Preisgarantie.

German usually writes compound nouns as a single word, where English often uses two words.

So English might say:

• price guarantee

But German combines the parts:

• Preisgarantie

This is completely normal in German. You will see this all the time, for example:

• Haus
  • Tür → Haustür
• Preis
  • Liste → Preisliste

Gilt is the 3rd person singular present tense form of the verb gelten.

So:

• gelten = to be valid, to apply, to be in force
• gilt = is valid / applies

Why gilt and not something like geltet? Because gelten is a strong verb and changes its stem vowel in the 3rd person singular:

• ich gelte
• du giltst
• er/sie/es gilt

In this sentence, the subject is die Preisgarantie (singular), so the verb must be gilt.

Both ideas are related, but gelten is very natural and common in German for rules, offers, tickets, guarantees, contracts, and similar things.

So German often says:

• Das Ticket gilt nur heute.
• Das Angebot gilt bis Freitag.
• Die Preisgarantie gilt bis Dezember.

You can think of gelten as meaning:

• to be valid
• to apply
• to remain in force

Ist gültig is also possible in some contexts, but gilt is often the more idiomatic choice for this kind of sentence.

Because with months, German often uses the month name without an article in time expressions.

So it is normal to say:

• im Dezember = in December
• seit Dezember = since December
• bis Dezember = until December

This is similar to English, where you also usually say until December, not until the December.

In bis Dezember, you do not see a visible case ending, because Dezember is being used as a bare month name without an article.

For learners, the most useful thing to remember is:

• bis is very common in time expressions
• with days, months, and similar words, German often uses no article

Examples:

• bis morgen
• bis Montag
• bis Dezember

If an article or adjective is added, the structure can change, for example:

• bis nächsten Dezember
• bis zum Dezember

So in your sentence, it is best to learn bis Dezember as a standard time expression.

Yes, you can, but bis Dezember is the simpler and very natural version.

• bis Dezember = until December
• bis zum Dezember = also possible, often a little more explicit or slightly heavier in style

Grammatically, zum is short for zu dem, so that version uses an extra preposition:

• bis zum Dezember = bis zu dem Dezember

In everyday German, with month names, the version without the article is extremely common.

Because German main clauses normally follow the verb-second rule.

The sentence structure is:

• Die Preisgarantie = first element
• gilt = finite verb in second position
• bis Dezember = the rest

So the basic pattern is:

• Subject + verb + other information

This is one of the most important word-order rules in German.

Because they are nouns, and German capitalizes all nouns.

Here:

• Preisgarantie is a noun
• Dezember is also a noun (the name of a month)

So both begin with a capital letter.

This is a major difference from English, where common nouns are usually lowercase.

The article tells you that here.

• die Preisgarantie can mean the price guarantee in the singular, because Preisgarantie is a feminine singular noun
• the verb gilt is also singular, which confirms it

If it were plural, you would expect:

• Die Preisgarantien gelten bis Dezember.

Notice the differences:

• singular: die Preisgarantie gilt
• plural: die Preisgarantien gelten