البوابة مفتوحة.

In Arabic, present-tense sentences like The gate is open usually do not use a separate word for is.

So:

البوابة مفتوحة
literally looks like
the gate open

but it naturally means
The gate is open.

This is very normal in both Standard Arabic and Levantine speech.

Because this is a full sentence, not just a noun phrase.

• البوابة مفتوحة = The gate is open
• البوابة المفتوحة = the open gate

That is a very important distinction.

In البوابة مفتوحة, مفتوحة is the predicate: it tells you something about البوابة.
In البوابة المفتوحة, المفتوحة is an adjective directly attached to the noun.

Because بوابة is a feminine noun, and Arabic adjectives must agree with the noun they describe.

So:

• masculine: مفتوح
• feminine: مفتوحة

Compare:

• الباب مفتوح = The door is open
• البوابة مفتوحة = The gate is open

A common clue is the ending ـة in بوابة, which often marks a feminine noun.

الـ is the Arabic definite article, meaning the.

So:

• بوابة = a gate / gate
• البوابة = the gate

In Levantine, الـ is usually pronounced something like el- or il-, depending on region and speaker.

A common Levantine-style pronunciation is:

el-bawwābe

A few helpful points:

• الـ often sounds like el-
• the وّ gives a doubled w sound
• the long ا is like aa
• the final ـة is often pronounced as -e in Levantine speech

So the whole sentence may sound roughly like:

el-bawwābe maftūḥa

Regional pronunciation can vary a bit.

Because ب is a moon letter, not a sun letter.

That means the l of الـ stays pronounced:

• البوابة → el-bawwābe

If the next letter were a sun letter, the l would assimilate instead.

Yes, here it functions like an adjective meaning open.

It comes from the root ف-ت-ح, which has the basic meaning to open.

Related forms include:

• فتح = he opened
• يفتح = he opens / opens
• مفتوح = open or opened (masculine)
• مفتوحة = open or opened (feminine)

So مفتوحة is the feminine form that matches بوابة.

Not exactly.

• باب usually means door
• بوابة usually means gate, main entrance, or gateway

So if you mean a normal house or room door, باب is more common.
If you mean a large gate, school gate, compound gate, or main entrance, بوابة fits better.

It is completely understandable and acceptable, and the spelling is standard.

In everyday Levantine speech, it can still sound natural, especially if you really mean gate. But for an ordinary door, people are more likely to say:

الباب مفتوح

So:

• البوابة مفتوحة = natural for gate
• الباب مفتوح = more common for door

A natural Levantine version is:

البوابة مش مفتوحة

Here مش is the common colloquial negation word.

So:

• البوابة مفتوحة = The gate is open
• البوابة مش مفتوحة = The gate isn’t open

You would remove الـ from the noun if you mean an open gate:

• بوابة مفتوحة = an open gate
• البوابة المفتوحة = the open gate
• البوابة مفتوحة = The gate is open

These three patterns are worth learning as a set, because they look similar but mean different things.