10ft, 20ft and 30ft are the full-size lengths of the actual cars represented by the models. Did you look at the pictures?
Please note that 1:32 is decidedly NOT 'G scale'. It is one of the original model scales posited before the end of the 19th C. Take a look at G1MRA. Models in Gauge 1, as it is called, run on 45mm gauge track and represent standard gauge operations running on 4ft 8.5in gauge track - iow, standard gauge, not narrow gauge - that's the remit of 'G scale'.
'G scale' is a catch-all term used to describe scales from 1:22.5 up to around 1:29*. The 'G' usually refers to the German word 'Gross' - 'big', from the Ernst Paul Lehmann producers of LGB - Lehmann Gross Bahn - Lehmann big trains. They mostly replicate the European metre-gauge stuff found in Germany, Austria and Switzerland. LGB are notorious for their 'rubber scale rule' - you'd need to look that up, but basically what at first glance might appear to be a half-way good model is distorted in any of the three dimensions - sometimes ALL of them.
*1:29 is a strange mélange of oversized models running on too-narrow track. The late Mr Polk, boss of Aristocraft, was unimpressed with the size and bulk of 1:32 scale, so he upped it to 1:29, running on the same 45mm gauge track, for what he called 'the WOW! factor'. USA Trains followed suit, and American large-scale was reborn. MTH in America remained staunchly 1:32, which enables their stick to run with ordinary 1:32 scale stuff without making it look undersized
Aristocraft disappeared about twelve years ago, but USA Trains thrives with a HUGE range of models - steam and diesel. Recently Bachmann Industries have re-introduced a few of the former Aristo diesels, at eye-watering prices, making ownership of the original Aristocraft models something to crow over - I have a lot of them.