I assume with gumtree they just ring you and can't actually buy on the site.  If this is the case I wouldn't bother saying anything about it being listed elsewhere.  When people contact you, just tell them the first one to hand over the money and pick up the item gets it, otherwise you could have people asking you to hold it for them so they can have first look at it and then never turning up.  If they can't actually buy it on the site, they'll have no idea who else has contacted you, and for all they know you could have sold it five minutes before they got there. 

I'd probably hold it long enough for someone to drive there (the time it takes for their trip) but no more.  We once advertised a fridge and several people wanted first option on it but none turned up.  We had to re-advertise it but we were smarter by then and said first one to show up with the money got it - and it sold it a few hours.

It used to be the rule on ebay that you couldn't list here and somewhere else at the same time, but I have no idea what the rules are now.  They keep changing things and it's hard to keep up, but I'd imagine the rule still applies because everything is about "the buyer experience".