The meanings of がる and みたい are different.
がる – an observation about how someone is feeling(from Tae Kim's page)
From the page：
怖がる act scared
嫌がる show dislike
使いたがる acts like wanting to use
欲しがっている be showing signs of wanting
行きたがってる be acting like (they) want to go to
But usages of がる are limited.
For examples with がる:
Using "he looks tired"
×疲れたがる be acting like (he/she/they) want to be tired ← no one voluntarily wants to be tired, so it's wrong.
×お腹すきたがる be acting like (he/she/they) want to be hungry ← Generally, no one wants to be hungry, so it's wrong.
×暖かがる wants to be warm ← Generally, we don't use 暖かがる, no one voluntarily raises own normal body temperature.
○寒がる show feeling cold
○暑がる show feeling hot
×筋肉質がる ← wrong
○筋肉質になりたがる be acting like (he/she/they) want to be muscular
○苦しがる ← According to an article, it shows a state of strong feeling agony, an emphasis of 苦しんでいる.
○苦しんでいる be in agony
×楽がる show feeling fun/better/easy ← Generally, no one voluntarily shows feeling fun/better/easy, so it's wrong.
○楽しんでいる having fun
○痛がる show feeling pain
Examples with "みたい"
怖いみたい seem to be scared
嫌いみたい seem to be dislike
使いたいみたい seem to want to use
欲しいるみたい seem to want
行きたいみたい seem to want to go to
*According to a study,
even describing がる's grammar for Japanese students seems to be difficult, but がる is often on JLPT N4 test.