Background: HER2 (ERBB2) overexpression or amplification (amp) are biomarkers for approved anti-HER2 therapies. ERBB2amp may be a superior predictor of anti-HER2 therapy outcome compared to IHC/ISH, and degree of CN gain may further stratify patients (pts). We investigated the distribution of ERBB2amp in adv GEA and hypothesized that increased CN was associated with better outcome to trastuzumab (T). Methods: Genomic analysis was performed using the Foundation Medicine (FM) tissue database (DB) of 313,896 pts with solid tumors including 12,749 pts with GEA and 34,629 pts with breast cancer (BC) used for comparison. ERBB2amp was defined as predicted CN ≥5 with > 80% of exons amplified. Using the nationwide US-based Flatiron Health-FM clinico-genomic DB linking de-identified EHR-derived clinical data to FM genomic data, pts with ERBB2amp adv GEA (01/2011 - 12/2020) were selected. RW progression (rwP) was obtained via technology-enabled abstraction of EHR. Multivariable Cox proportional hazard models were used for outcome comparisons. Results:ERBB2amp was detected in 15% (1,920/12,749) of GEA samples; median CN 22 (IQR 9-73), and 97% of cases had full gene amp. Median ERBB2 amplicon size was 0.27Mbp (IQR 0.13-0.95). In comparison, ERBB2amp was detected in 9.2% (3,193/34,629) of BC samples; median CN 20 (IQR 9-40), median amplicon size 0.32Mbp (IQR 0.13-1.37). In both cancers, smaller amplicons were associated with higher CN (P < 0.001), excluding amplicons < 0.1Mbp where less than 100% target amp was common. ERBB2amp was additionally seen in 2.7% of other solid tumors, and specifically in 2.3% of NSCLC and 3.1% of CRC. In the RW DB of 183 pts with ERBB2amp adv GEA, chemo + T (45%) and chemo alone (17%) were the most common first therapies after genomic report. In 101 evaluable first-line T treated pts ERBB2 CN was a significant predictor of rwP free survival (rwPFS) as a continuous variable (aHR = 0.74 [95% CI: 0.61 - 0.89], P = 0.002) and a range of cutoffs were similarly predictive. For control, in ERBB2amp pts treated with chemo ERBB2 CN was not predictive of rwPFS (aHR = 0.93, [95% CI: 0.72 – 1.20], P = 0.060). Among T treated pts, co-PIK3CA mutation was more common with lower CN (p = 0.03 by Wilcox test); no significant differences were observed for primary tumor location, age, stage at adv diagnosis, co-KRASmut, EGFRamp or FGFR1/2amp. Conclusions:ERBB2amp was detected in 15% of GEA tissue samples, with significant diversity in ERBB2 CN and amplicon focality, but with a similar CN distribution and amplicon focality seen in ERBB2amp BC. ERBB2 CN was predictive of rwPFS as a continuous variable for pts with GEA treated with T in the RW setting. Further studies exploring the clinical utility of quantitative ERBB2 CN, and extending to ctDNA, particularly in the setting of the evolving anti-HER2 landscape and combination therapies, are warranted.