# surely bless

"certainly bless"

# I will greatly multiply your descendants

"I will cause your descendants to increase again and again" or "I will cause your descendants to be very many"

# as the stars of the heavens, and as the sand which is upon the seashore

"beyond what you can count" 

# as the stars of the heavens

Here the word "heavens" refers to everything we see above the earth, including the sun, moon, and stars.

# will possess the gate of their enemies

"will completely triumph over their enemies"