The Open Font License is a free software license, and as such permits the fonts to be used, modified, and distributed freely (so long as the resulting fonts remain under the Open Font License). However, the copyright holder may declare the font's name as being a "Reserved Font Name", which modified versions then cannot bear. The License permits covered fonts to be freely embedded in documents under any terms, but it requires that fonts be packaged with software if they are sold.
The Open Font License is incompatible with the GPL due to its strong copyleft. Even if it did not have a strong copyleft, it would still be GPL-incompatible due to its restrictions on the Reserved Font Names and selling the fonts, which are considered "further restrictions" within the meaning of section 10 of the GPL.