がる is a う-Verb that is primarily used as a suffix in Japanese. When used in this way, it means ‘to act like (A)’, or ‘to show signs of being (A)’, where (A) is the word that it is attached to.
To use がる, you will need to link it to the stem form of an い-Adjective (simply remove the い), or to the stem form of a な-Adjective (do not include the な).

• 彼（かれ）はいつも女（おんな）の子（こ）の前（まえ）では強（つよ）がるけど友達（ともだち）の中（なか）では彼（かれ）は一番（いちばん）の怖（こわ）がりだ。
  He always acts tough in front of girls, but amongst our friends he is the biggest scaredy-cat.
• 犬（いぬ）が嫌（いや）がる事（こと）をしてはいけない。
  You must not do anything that the dog would not like.

がる is a verb which means ‘to give the impression of (A)’, or ‘to act like (A)’. This is very important in Japanese, as you would not usually say how somebody else feels, unless they told you directly and you are simply repeating what you heard (in which case you would use the quotation particle と).

• トムはこの新（あたら）しいゲームを欲（ほ）しがると思（おも）うよ。
  I think Tom would want this new game.

When you want to express the way you ‘think’ that someone else feels, but don’t have any direct information about it, that is when がる would be natural to use. For example, if you see someone shivering in the cold, you would say something like the following.

• あの人（ひと）、寒（さむ）がっている。ジャケット着（き）ればいいのに。
  That person seems like they are cold. It would be better if he wore his jacket. (Natural Japanese, as がる shows how he ‘appears to be acting’, rather than what he is actually feeling)

The main difference between がる, and がっている, is that がる will be used when a person usually/always acts a certain way, whereas がっている is more about the way someone/something is acting in any specific moment.

• この子（こ）はいつも新（あたら）しいおもちゃを欲（ほ）しがる。
  This kid always wants new toys.
• キムは新（あたら）しいパソコンをほしがっている。
  Kim is wanting a new computer.