Sellars, 21, left Aston Villa at the end of last season and has experience of the fourth tier after a loan spell with Wycombe in 2015-16.
Sudan-born Eisa, 22, scored 57 goals for Greenwich over two seasons.
"We've known about Mo for a couple of seasons and kept our eye on him due to his goalscoring prowess at that level," said boss Gary Johnson.
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League Two club Cheltenham Town have signed forwards Mohamed Eisa and Jerell Sellars, both on one-year deals.