が出来たら

if/when ... is ready or completed

→ が
phrase Rare #31,741
if/when ... is ready or completed 出来たら is the conditional form of 出来る. Depending on context, it can also mean “if ... develops” or “if I/you can ...”; が marks what is ready, completed, developed, or possible.
JA: 準備が出来たら、出発しましょう。
EN: When we're ready, let's leave.

もし腫瘍が出来たら

If a tumor develops

— ファイト・クラブ (1999)

先生は用意が出来たら私たちを呼んで下さるでしょう

The teacher will call us when they’re ready.

私も彼のように遠く泳ぐが出来たらいいのに

I wish I could swim as far as he can.

先生は用意が出来たら私達を呼んでくれるでしょう

The teacher will call us when they’re ready.

車の運転が出来たらどんなよいかと私は思った

I thought how wonderful it would be if I could drive.

書類が出来たら提出しろ

Submit the documents when they're ready.

— 預言者 (2009)