Like currently there is gaps on most of major pairs in currencies, also gold, but I see its continuous up trends on them, that is ok , but I thought you meant trying to predict gap on Fridays before the market closes.
Depending on account size. Account of over a million dollar, I will not risk more than 5% if i seem too over sure and want to go for all, account of 100k and 500k, I have risked max 10% on both, and it was a good pay as I made over 37%.
You should always have target of what you can lose. You can allocate 1 to 3 percent of total account risk to all positions. The bigger the account, the lower the risk applied for me.