GSM-7 is the default 7-bit SMS alphabet: 128 basic characters covering Latin letters, digits and common punctuation, allowing 160 characters in one message (153 per part when concatenated).
GSM-7 packs 7-bit characters into the 140-byte payload of an SMS, which is how 160 fit in one message. The basic table holds the Latin alphabet, digits, common punctuation, a few accented letters (é, ñ, ü, à, ö…) and some Greek capitals. A second table of extended characters (such as the euro sign, square brackets, braces, backslash) costs two positions each.
One character outside both tables, a curly quote, an em dash, an emoji, a Cyrillic or Arabic letter, switches the whole message to UCS-2 and halves its capacity. That is the single most common cause of an unexpectedly doubled campaign cost.
At SMSMeteor: the composer and the API response report the encoding and segment count before anything is sent; GSM-7 holds 160 characters (153 per part when concatenated).