Are you talking about this 3g eclipse? The tank is only 12" long, and I'd guess your boy is about 2.5-3" long... not very far to swim.
I wouldn't put a dwarf gourami in anything smaller than a 5g rectangular tank this one has a seamless look which is popular these days, and it's 16.5" long.
But if you have to decide between a 0.5g "tank" and the 3g, please upgrade to the 3g. I suggested the 10g because you said you already had a couple in storage, which wouldn't cost you anything to set up.
I wouldn't put a dwarf gourami in anything smaller than a 5g rectangular tank this one has a seamless look which is popular these days, and it's 16.5" long.
But if you have to decide between a 0.5g "tank" and the 3g, please upgrade to the 3g. I suggested the 10g because you said you already had a couple in storage, which wouldn't cost you anything to set up.