I don't have all of the 1930s ORERs, but I have enough to be able to sketch out the basics of the GATC fleet growth via acquisition. Note that many of the mid-1930s ORERs did not have car counts from GATC (and other lines) anyway; this is another instance of when Ian's reprint of the 1936 tariff can really help. So, briefly:
1. As far back as 1919, you can see GATC scooping up mom-and-pop leasing companies (Adler Tank Line, 6 cars) and refinery fleets (Kanoky Oil, 1 car). In my 1919 edition, there 11 reporting marks other than GATX listed. These tended to come and go over time, but I don't think in the aggregate we are looking at more than a few hundred cars in the 1920s.
2. Things get more interesting between 12/30 and 7/35 with acquisition of the Standard Transit (STCX) and Quaker Tank Line (QTX) fleets, about 3000 cars each. (Also Swift Tank Lines, but only ~400 cars). These reporting marks are gone sometime after 1940, but before 1945.
3. Probably early in 1936, Proctor and Gamble (PGX) was also added -- ~1200 cars. This reporting mark has disappeared by January, 1940. About the same time, the Canton Tank Car Co. (CTTX) was also added -- about 780 cars.
4. Also by early 1936, the merged Pennsylvania-Conley Tank Lines (PTX, CYCX) had purchased the Texaco (TCX) fleet for lease-back, and their total holdings reached ~14,300 cars. GATC had at least some control of this enterprise dating back to maybe 1928 (?; it's murky), but the separate P-C listing in the 1936 tariff doesn't mention the GATC "parentage". This becomes explicit by the time of my 1/38 ORER.
5. Some time between 1940 and 1945, the separate Penn-Conley listing disappears, and all three reporting marks appear under GATC's banner in the ORERs. [I did not look past 1945].
I think those major acquisitions explain quite a bit (but certainly not all) of the growth of the GATC fleet totals from (very round numbers) 12,000 in 1930, to 25,000 in 1936, to 38,000 in 1945. Of course there may have been (and probably was) concurrent scrapping of old, and building of new, cars but that will prove difficult to tease out with what we have to work with.
With best regards.
Swall Meadows, CA