If you can't use the gondola you can't easily get up to the runs in Bansko. There is a ski road which you could walk but it is 7km long. Suggest you take this up with your tour op if it is true. I was there last year at the same time as you and it was not closed.
been checking on various sites about the bansko ski lifts and plenty of them mention the closure days for the borovets ones but non seem to mention any closure days for the bansko ones - you might be lucky.
If not and they are closed as Val says take it up with your travel company they should be warning you in advance
Bright side - plenty of if you don't have to get up for skiing!!
I've never heard of the gondola in Bansko being closed for maintence,(and I've skiied plenty of Mondays this season) but if it is you would access the slopes the way they did before the gondola was put in, by bus up to shiligarnika!!!