Worldwide mobile app download revenues stood at $18 billion in 2012 and 64 billion mobile apps were downloaded during the same period, it said in a statement.
"Mobile app stores will see annual downloads reach 102 billion in 2013, up from 64 billion in 2012. Total revenue in 2013 will reach $26 billion, up from $18 billion in 2012," Gartner said.
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Free apps will account for 91% of total downloads in 2013, it added.
"We expect strong growth in downloads through 2014, but growth is forecast to slow down a bit in later years," Gartner Research Director Sandy Shen said.
Average downloads per device should be high in early years as users get new devices and discover the apps they like. However, over time they accumulate a portfolio of apps they like and stick to, so there will be moderate numbers of downloads in the later years, Shen added.
"iOS and Android app stores combined are forecast to account for 90% of global downloads in 2017. These app stores are still increasingly active due to richer ecosystems and large and very active developer communities," Gartner Research Director Brian Blau said.
However, the firm expects average monthly downloads per iOS device to decline from 4.9 in 2013 to 3.9 in 2017, while average monthly downloads per Android device will decline from 6.2 in 2013 to 5.8 in 2017, he added.
This relates back to the overall trend of users using the same apps more often rather than downloading new ones, Blau said.