Hire of baby gate

You can buy a stair gate new on Amazon or Ebay for about €29 and another €10 postage to send it to your place in Spain.  Consider purchasing a secondhand one locally to where you live and posting it to Spain.

Leave it behind or drop it into a charity shop when you're finished. 

For  me I think the time and bother seeking a rental stair gate, (ensuring it will fit safely at your staircase, returning it, leaving a deposit or your credit card details etc), just doesn't justify the €10 or so that you might save.

For free you could make an obstruction at the top or bottom of the stairs such as an upturned chair - ideally with with colourful distractions like favourite toys prominently visible well away from the stairs.  

If this is in place from the immediate outset your child will accept it as normal. That's what we used to do when ours were that age.