We've stayed at the forestry site in Beddgelert on many occasions and must admit I cannot recall being bothered by midges before. We've stayed at all times of the year and it is great for walking in the Snowdonia area and Beddgelert itself is a lovely village.
We were bothered by Midges however in June '13 at the site up the road at Bryn Gloch but this maybe because the site is located in the open air and has a river running around the site. Therefore they must be in the vicinity and maybe we have been fortunate not to encounter them at the Beddgelert site, although I will admit that I am not an expert on their life cycle and their preferred habitat.