Gaps and limit/stop orders

Let's say I set a limit or stop order at a certain price - and there is a gap that overlaps the price - does the trade activate?

Yes. Your trade will be activated because the open price AFTER the gap will already overlap your limit. Doesn't matter that the price went up/down because of a gap.

You write a simple piece of code will prevent that on MT4

Actually, don't count on it for a limit order. Stop order, no question......along with consequent slippage.

The price you see on a chart is the Bid price, so if you have a buy limit order(pending order at below market price), you may have the bid price open up with a gap well below your target price......typically gaps are caused by poor weekend liquidity(which also causes big spreads) and there is a chance that the Ask price(which is the price your order needs for a buy) may still be above your order price.