GTM-v2-base by OpenGCM

A ~120M parameter, decoder-only GPT-style base language model, trained from scratch on a single RTX Pro 6000, on roughly 4 billion tokens streamed from a mix of open web/educational/math corpora.

This is a base (pretrained) model, not an instruction-tuned or chat model. It completes/continues text; it does not reliably follow instructions or answer questions directly. It also has no fine-tuning for factual accuracy — it can and does confidently generate fluent, plausible-sounding but factually incorrect content. Treat it as a small, from-scratch research/hobby model, not a production system.

A separate instruction-tuned variant (SFT on UltraChat) is planned as a follow-up release; this repo is the base-only checkpoint.

Model details

• Architecture: nanoGPT-style decoder-only transformer

  • 14 layers, 8 attention heads, 704 embedding dim
  • ~119.4M parameters (weight-tied embeddings/output head)
  • Context length: 1024 tokens
  • Uses PyTorch's fused scaled_dot_product_attention (flash-attention kernel)

• Tokenizer: tiktoken GPT-2 BPE encoding (tiktoken.get_encoding("gpt2")), vocab size 50,257. No custom tokenizer was trained.

• Optimizer: Muon (for 2D weight matrices) + AdamW (for embeddings, layernorms, biases) — a hybrid setup, following the approach popularized in recent efficient-pretraining recipes.

• Precision: trained with bf16 autocast; released weights are fp32.

• Training tokens seen: ~4.16B tokens (40,690 steps x effective batch 100 x 1024 context)

• Training data: streamed via HuggingFace datasets, mixed and tokenized on the fly (no fixed local copy of the source datasets):

  • FineWeb-Edu (HuggingFaceFW/fineweb-edu, sample-10BT) — 45%
  • Cosmopedia-v2 (HuggingFaceTB/cosmopedia-v2) — 30%
  • FineMath (HuggingFaceTB/finemath, finemath-4plus) — 15%
  • FineWeb (HuggingFaceFW/fineweb, sample-10BT) — 10%

  Note: code data (The Stack v2, StarCoderData, the-stack-smol) was intentionally left out of this mix — every BigCode-hosted code corpus we checked is gated behind a terms-of-use click-through on HuggingFace, so none of them are pulled in without the user separately accepting those terms and authenticating.

This is the second model in this series. An earlier ~100M parameter version (GTM-v1-base, trained on ~3B tokens with the same data mix and optimizer) informed several of this model's design choices, including the Muon/AdamW optimizer split and the corpus weighting above.

Benchmarks

Evaluated via likelihood-scoring (comparing the model's per-token loss across candidate answers, no generation/sampling involved), 200 examples per benchmark, final checkpoint at step 40,690 (~4.16B tokens seen):

Benchmark	GTM-v2-base	GTM-v1-base	GPT-2 (124M)	Random baseline
HellaSwag	33.0%	32.0%	~28-29%	25%
ARC-Easy	43.0%	42.5%	~39.2%	~25%
ARC-Challenge	24.0%	26.0%	~22.5%	~25%

Corpus perplexity (per-source, lower is better) on freshly streamed, non-training-shard text:

Source	Perplexity
Cosmopedia-v2	11.80
FineMath	13.04
FineWeb-Edu	27.68
FineWeb	47.34
Overall	21.19 (vs. GTM-v1-base's 22.87)

GTM-v2-base improves on GTM-v1-base (an earlier ~100M param, ~3B token version of this same series) on HellaSwag, ARC-Easy, and overall perplexity, but is not a clean sweep — it's slightly behind v1 on ARC-Challenge (24.0% vs. 26.0%), though both remain above GPT-2's ~22.5% reference. ARC-Challenge is a small (200-example), noisy evaluation at this scale, so a few-point gap in either direction shouldn't be read as a strong signal on its own.

GPT-2 (124M) reference numbers are from a from-scratch reproduction verified to match official GPT-2 evaluation results. GTM-v2-base was trained on roughly 1/10th of GPT-2's training tokens (~4B vs. ~40B), on a single consumer/workstation GPU rather than a large multi-GPU cluster.

These numbers should be read as "beats a 2019 baseline on a few narrow multiple-choice benchmarks," not "as good as or better than GPT-2 in general." GPT-2 was trained and evaluated far more broadly; free-form generation quality, factual grounding, and behavior outside these specific benchmark formats have not been rigorously compared.

Known limitations

• No reliable factual recall. E.g. prompted with "The capital of France is", the model does not consistently produce "Paris" — it may produce fluent but factually invented content instead. This is expected: the training corpus is not dense in discrete factual content, and this model's parameter/token budget is small for reliably memorizing specific facts.
• No code capability. No code-specific training data was included (see above). The model can produce code-shaped text (recognizing "write code" prompts and using plausible syntax) but not functionally correct code.
• Not instruction-tuned. Does not follow instructions or answer questions in a chat-like way — it continues text. (A separate SFT/chat release is planned; this is the base-only checkpoint.)
• Repetition tendency. Base generation (greedy or low-temperature sampling) can fall into repetition loops or lock onto structural templates (e.g. numbered-list/fill-in-the-blank formatting) when it lacks a confident continuation. A repetition penalty (see usage example) substantially reduces this.

Usage

Requires model.py (included in this repo) alongside the checkpoint — this is a plain PyTorch model, not a transformers AutoModel.

pip install torch safetensors tiktoken

import json
import torch
from safetensors.torch import load_file
from model import GPT, GPTConfig

with open("config.json") as f:
    cfg_dict = json.load(f)
config = GPTConfig(
    vocab_size=cfg_dict["vocab_size"], block_size=cfg_dict["block_size"],
    n_layer=cfg_dict["n_layer"], n_head=cfg_dict["n_head"],
    n_embd=cfg_dict["n_embd"], dropout=cfg_dict["dropout"], bias=cfg_dict["bias"],
)
model = GPT(config)
state_dict = load_file("model.safetensors")
model.load_state_dict(state_dict)
model.eval()

import tiktoken
enc = tiktoken.get_encoding("gpt2")

prompt = "Once upon a time,"
ids = enc.encode_ordinary(prompt)
x = torch.tensor([ids], dtype=torch.long)

with torch.no_grad():
    out = model.generate(
        x, max_new_tokens=128, temperature=0.8, top_k=50,
        eot_token=enc.eot_token, repetition_penalty=1.3,
    )

print(enc.decode(out[0].tolist()))

License

Apache 2.0 for this repo's contents (model weights, model.py, and this README). The underlying training data retains its own licenses regardless of the license on this trained model — FineWeb, FineWeb-Edu, Cosmopedia-v2, and FineMath are each ODC-BY-1.0 (see their respective HF dataset cards for full terms and attribution requirements). This repo distributes model weights, not the training data itself.